I don't mean to rain on the hope that the Democrats might reach the magic number of 60 -- I really hope they do -- but I thought I should point everybody's attention to two other Senators that nobody has mentioned yet: Daniel Inouye and Daniel Akaka.  Both named Daniel.  Both Democrats.  Both from Hawaii.  Both 84 years young (and born only 4 days apart).
 
Now, obviously they are still in good health and could easily serve out their terms with no cause for concern.  However, they are getting on in their old age, and should either of the Daniels be unable to serve or (heaven forbid) die in the next couple of years, they would likely be replaced by a Republican Senator because Hawaii's governor, Linda Lingle, is a Republican.  It also warrants mentioning that Lingle's term will be up in 2010 and she could be succeeded by a Democrat.  But, she might just as easily run for the Senate in 2010 against Inouye -- or run for an open seat if he retires -- and given her popularity among Hawaii voters, she could win it.
 
Just to carry the argument even further, Dianne Feinstein of California is 75, and Schwarzenegger could replace her with a Republican if she were unable to serve.  Of course, that's balanced out by Arlen Specter of Pennsylvania, who's 78, represents a state with a Democratic governor, and will be up for re-election against (possibly) Chris Matthews or the governor himself.