We're in April 2007, the election is in November 2008... Things change.
Very true.
I don't blame the Dems for being optimistic, though. But generally, big swings like we had in 2006 cool off somewhat in the next cycle. Of course, the Democrats actually added one seat in 1976 on top of their monstrous gains in 1974, so who knows.
31 seats was really not that big of a swing. The big swings happened in 1994(54 seats), 1974(47 seats), 1966(47 seats), 1958(49 seats). However, we rarely see swings any higher than 10 seats these days due to incumbency advantages.