With the Democrats apparently picking up 6 seats, I wondered if the particular third of Senators being elected were a Republican-heavy group, and thus easier to make gains from. It actually appears to be the opposite; this is the one of three classes the Democrats were already leading in.
Class 1 before
15 Republicans
1 independent
17 Democrats
Class 1 after
9 Republicans
2 independents
22 Democrats
Class 2 currently, next election 2008
21 Republicans
12 Democrats
Class 3 currently, next election 2010
19 Republicans
15 Democrats
More interesting would be a look at how close these races have been in the past, but that's more than I feel up to right now. I know 2 years ago (Class 3) most of the Republican Senate gains were by a pretty small amount, but it looks like this year's close victories have tended the other way.