Ed, in looking at the past numbers on the right, I don't see anything particularly shocking about the current snapshot. In fact, it says that (R) candidates had their first lead over (D) candidates 12 months ago!
Maybe passing health care actually energizes a few more Democrats. Progressives have been demoralized for a while now, which does a lot to explain the continued slide since Obama's election.
I think there are enough progressives to keep a Democratic majority, and the only way for a big win by the GOP is if Democrats stay home on election day. And it will depend a lot on whether Obama can deliver more legislation this year.