Actually, even if the Dems lose the MA seat, they can still get HCR through. Here are some ways...
If the Senate can vote on HCR before Brown (assuming he wins) gets certified & seated, then it's a done deal - I think this could very well happen because with the Dems controlling the whole state government of MA, they could easily stretch out the time to certify the election, either legitimately or by less-than-believable means. Another potential wrinkle is that the loser of the MA election could challenge the winner in court, which could put the election results in limbo and therefore allow the Senate vote to go through. My guess is that the second route (challenging the results in court) is the more likely path.
Another way is to simply have the House vote on the Senate version of the bill as it is currently written. A lot of liberals in the House would have to hold their noses to do it, as the Senate version is less liberal, but if push came to shove I think they would. Better, at least politically for them, to have that than nothing at all.
A third way, which I see as very unlikely though stranger things have happened, is to cut some kind of deal with a moderate Republican in the Senate (probably Olympia Snowe) to provide the critical 60th vote. This would, I think, be an absolute last ditch effort, but it isn't outside the realm of possibility.
So HCR isn't dead if the Dems lose the MA seat, it'll just be a bit more tricky to get it through.
Btw, I heard on the radio today that Obama will be giving his State of the Union speech on Jan. 27th. This makes me think they'll be going with option 1 or 2 above so that he can say they got HCR done in his speech. He'll want to say "we got it done" so that he can move on and discuss other aspects of his agenda, because even us proponents for his HCR plan are starting to get sick of hearing about it all the time.