I think this analysis from Nate Silver about the probability that the health care bill will pass (he's at about even-money) is too pessimistic on at least two counts.
First, the picture looks a lot better if you make the likely assumption that Pelosi had a fair number of extra votes lined up, but that she didn't need to call on because she already had enough for passage, and the members in question felt that if they weren't needed they'd rather not stick their necks out. The vote total for the bill fits this interpretation pretty well. She needed 218, but if she got exactly 218 everybody would've been 'the deciding vote to pass health care reform' and some people from marginal districts probably didn't want that. Add one more to avoid that problem and then the Republican Anh Cao, who she might not have felt she could count on, and you've got the 220 she had. This suggests the possibility that there were a few members who said, "Okay, Nancy, I'll be there if you need me, but I'd rather not be seen doing this in case the Senate can't pass the bill and it ends up being a Thing That Failed."
Second, Nate's point that "nobody who voted against the bill before has yet affirmed that they'll switch to vote for it" is less significant than he thinks. Once you declare yourself in favor, you lose your leverage in future negotiations. This is especially important in light of his concern about how none of the Blue Dogs are being drawn in by the more conservative Senate bill. Why would they come out early in support of the legislation and give up leverage?
Sidenote: it's one of the bizarre features of the Senate negotiations that the people who exercised maximum leverage basically used it in ways that ruined their public image -- Ben Nelson's Cornhusker Kickback was roundly derided, and Joe Lieberman dropped dramatically in the polls after killing the public option. Hopefully no Blue Dog will come out and demand, I don't know, a big naked statue of themselves in exchange for voting yes.