I think Kevin Drum has the electoral consequences of health care for 2010 just about exactly correct. Republicans are now predicting (hoping?) that voting for health care will doom those Democrats who will be on the ballot in eleven months, but as Drum points out, November is a long time from now, and most of the provisions aren't going to affect anyone in any serious way in 2010.
I'll add one thing that I know, and one that is speculative. What we know is that votes on single issues rarely make any difference in general elections. Partisan affiliation is by far the most important factor in voting for Congress, and that's not going to be affected by any vote, and certainly not one that breaks along partisan lines, like this one. The second important thing is, roughly, the difference between evaluations of the candidates, which is driven by incumbency and (to a lesser extent) campaign spending (the big gap is that most people in most House races never even hear about the out-party candidate, which has nothing to do with specific issues. Basically, Republican challengers who can raise tons of money need something to say, and the health care vote might serve that function, but what's important to election outcomes is that Republicans recruit good candidates who then find plenty of funding. Beyond those two major factors, everything else -- popularity of the president, the economy, campaign quality, and on down to positions on specific issues of public policy -- are minor or marginal at best.
(I should clarify -- that's the direct effect of those things. If Democrats think it'll be a bad year because of the economy or health care and retire, then it has a large effect, mostly played out through the lack of incumbency advantage).
OK, now the speculative part, which is about effects on the margins. To the extent that specific issues matter (which, again, and it can't be emphasized enough, is usually not very much), I do wonder whether GOP overpromising will hurt them. If one listens to Republican rhetoric, one would think that most people will get hit with a whopping tax increase beginning the minutes Obama signs the bill into law -- and that Medicare recipients would lose most of their benefits at the same time. Since that will not, in fact, be what happens, I wonder if Democrats will benefit from the absence of cataclysmic short-term consequences. Of course, that will be balanced off by GOP accusations that anything bad in health care at all is a direct function of the new law...it goes back to that Republicans will perceive the law as harmful, and Democrats will perceive it as helpful. I do wonder, however, if pre-passage extreme rhetoric has any effect at all on post-passage impressions.