Not exactly, depending on how you interpret the essence of the contract.  If you understand the credit card agreement to be a contract for a continuing service, then you're not quite correct.  For example, if I sign up for a card that will extend me a $10,000 credit limit under X,Y,Z terms, during a period with an expiration date of 12/08, then that cardmember agreement constitutes the contract for that period of time.  Both parties are bound by X,Y,Z terms.  Simply using the product/service according to the terms of the contract (by making purchases, paying monthly minimums, etc.) does not mean you are forming a new contract each time you do so.  The offer and acceptance - essentially, you filling out the mail solicitation, mailing it back, being approved and then activating the card - has occurred, and the contract is formed.  Without the change-in-terms clause, credit cards could make offers to form a new contract or revise the old one, but they couldn't say that you merely exercising your rights under the original existing contract (i.e. making purchases) would equal your acceptance of their offer. 

Imagine for a moment that I offered to give you my stereo if you paid me $100/month for the next ten months.  Then, during month two of the contract, I sent you a letter saying that I would like you to instead pay me $200/mo. for the remaining eight months.  At the same time, I tell you that if you subsequently make a $100 payment after receiving my request for more money -- as you are required to do under the existing contract -- you are agreeing to my new terms.  Do you see the problem?  Your choice is either to breach your contract or agree to my unilateral proposal against your will.  I cannot coerce you into agreeing to my offer in this manner; likewise, I can't simply choose to disregard the contract to which I'm already bound.

I like to compare credit cards to cell phones.  Let's say you sign up for a two-year service contract with Verizon.  For the basic terms, you agree to pay $50/mo. for 1,000 anytime minutes per month.  Under the contract, you are both bound by these terms for two years -- Verizon must provide those minutes of airtime, and you must pay that monthly amount.  Now let's say Verizon decides to pull in a little extra cash by sending an insert with your monthly statement announcing that, from then onward, your cost will be raised to $75/mo. for only 500 anytime minutes.  Verizon can't simply disregard your existing contract; this rate change constitutes an offer from the company -- an offer which you clearly will not accept.  But, what if Verizon also states that anyone placing or receiving a phone call going forward agrees to the new rates?  That doesn't work.  The company has a contractual obligation to provide you with certain airtime minutes, but you have a corresponding right to receive and use those minutes under the existing terms of your contract.  Verizon couldn't alter your contractual right any more than it could alter its own contractual obligation. 

Moreover, if you read my prior post on this subject that (linked to above) you'll notice an even bigger problem: the changed contractual terms typically apply retroactively to balances you already carry.  Your comment references this point and suggests that it should be against the law, but it currently is considered acceptable practice.  In other words, if you have a $5,000 balance and the credit card changes your terms to say that your 0% promo rate is bumped to 29%, your late fees increased twofold, and your minimum monthly payments doubled, these changed terms apply to the balance you already carry.  Using our imagination again, pretend you went to the store and purchased a CD for $10.  How would you respond if the store called you a week later and demanded an extra $5 for the CD long after it was purchased?  Applied to credit cards, imagine you want to purchase a $3,000 flat panel television but don't have that much cash on hand.  You calculate that you can afford an additional $250/mo., so you sign up for a credit card with a 0% promotional APR for one year and purchase the television with the expectation of making those payments for twelve months until the balance is paid down.  One month in, your credit card company terminates your promo rate and hikes your standard APR to 20%.  Suddenly, your purchase price for the $3,000 television just jumped to $3,600 (ignoring compounding penalties and fees), long after you made your purchase. 

One final point.  I would argue that this isn't a contract at all.  If every single term of the contract is subject to be altered or removed at any time, upon the unilateral action of one party, then what terms did I agree to?  The law has a name for this, and it's an "illusory" contract.  An illusory contract appears to be a contract but in fact is not legally enforceable because it lacks at least one essential aspect of a contract; in this case, it's lacking all aspects of a contract because there isn't a single term written in stone that can't be changed at the whim of the party.  In the simplest sense, this is no different than a "contract" that states only "I'll give you $10 if I feel like it," which is no contract at all.

If I were going to sum up a credit card contract beneath the Ad for it, it would go like this:

AD:

GET WHATEVER YOU WANT, WHEREVER YOU WANT, THE WAY YOU WANT IT! (insert: picture of someone really fit wearing danskins, high-end boots and clothes, field hat, SUV keys on belt, GPS-cell phone-PDA-cuisinart-lie detector), sipping Pomegranite wine, and meditating at the top of Mt. Everest with an open laptop, and a bunch of wasted Sherpas laying all around with her gear on their backs).

(less conspicuously, the contract language:)

SYDB (SuckYouDryBank), hereafter, laborless black hole, for evil and unreasonable consideration, and based on special lobbying access you'll never have, and historical events outside of your control, contracts with YOU, (in the accusatory sense, the Consumer Pig who wants too much) as follows:

Whereas, YOU have little or no money to your name significant enough to talk about, due to the historical events outside of your control referred to above that helped put you where you are today and us where we are today; YOU in your current state of uneven bargaining power, fog, ignorance, low income and daily anesthetic temptation in viewing the beautiful things on which billions are spent (more than the history of education funding in your state) to convince you we're your ticket to those things that money can buy, hereafter, (HAPPINESS) and;

Whereas, SYDB has obscene amounts of funds sitting around gathering interest and has had same since its inherited seed money which came from endeavors of past generations who paid our founders interest to survive, AND came from the incredible boon of free labor inherent in the most convenient windfall of that tsk-tsk aweful slavery thing in the beginning AND;

Whereas, SYDB has a large collections department to pay for, and

Whereas SYDB and its collections department is more important than your child's nutrition, education, intrinsic humanity, spiritual worth, and physical usefulness, well, because look at YOU; and

Whereas YOU hereby AGREE and ACKNOWLEDGE and BIND yourself as admitting our definition of YOU and the utter rightfulness of SYDB as defined herein; and

Whereas YOU hereby AGREE and ACKNOWLEDGE and BIND yourself and your children's future to what SYDB wants from you, hereafter THE RATE OF BLOOD, SWEAT & TEAR FLOW;

YOU AGREE and ACKNOWLEDGE and BIND yourself to PAY us, SYDB whatever we want. when and where we want it, and the way we want it because YOU need it. Should YOU sign below, you already AGREE to give us HAPPINESS. Thank YOU for being our happy little Sherpas.

SYDB.

_____________________________________________

YOU

I may have been understating the problem above. I want to apologize for my reductio treatment of credit card abuse by lenders with the caspar milktoast language I used in that fictional contract.