I wish he wouldn't do that, but I'll look for a silver lining or two that might come out of it.
First of all, Obama's response showed yet again that he faces attacks from the right and the left with great skill.  Check out this great blog entry from Ghengis:http://tpmcafe.talkingpointsmemo.com/talk/2008/02/obama-on-nader.php

Second, as a moderate/independent, I'm not unhappy about having the sort of people who would vote for Nader filtered out.  It's not a matter of party loyalty (I don't believe in party loyalty because if you reward bad decisions by the party you simply get more bad decisions).  And it's not that an independent candidate couldn't run a credible campaign even within the two-party system we're stuck with. 
It's that Nader's explanation for why he's running makes no sense.   Yglesias said it very well:http://matthewyglesias.theatlantic.com/archives/2008/02/tweedlesomething.php
<blockquote>After all, there's not a dime's worth of a difference between a candidate promising tax cuts, pushing more health risk onto individuals, a re-invigoration of George Bush's campaign to dominate the world through military force, and an industry-friendly approach to environmental issues and his rival who's promising substantial socialization of medical risk, a 80 percent reduction in carbon emissions, and end to the war in Iraq (and to the mindset that led to war!), universal preschool, etc. Well, sure, there's judicial appointments -- abortion, gay rights, etc. -- and some small fry stuff about whether or not the NSA will have unrestrained surveillance powers. But basically it's just the same two corporate clones running on virtually identical platforms.
</blockquote>
Siphoning off the sort of voter who would fall for that is really doing the Democratic party a favor.   They're the lefty equivalent of the wingnuts.
Third, siphoning off those voters will tend to move the Democratic party toward the middle.  As a moderate/independent, I welcome this shift.  
And if the Democratic party moves toward the middle, then where does someone like McCain go except toward the hard-core right.   But he can't satisfy the wingnuts, and nobody really can because they're not even close to being rational.  They fall for flimsy rationales for all sorts of things, but while they are sometimes easily manipulated, at other times they're hard to predict.   
And maybe someone will take advantage of that, a Dobson-approved candidate for the wingnut vote.  Nader getting into the race makes that even more likely.   


In summary, I'd much rather Nader wouldn't run because it adds some risk that McCain will win, and what could be stupider than doing that?  It's even stupider than Nader helping Bush win!  But given that his ego is going to compel him to run, we might as well look for some silver linings.