see my #21
My reasons for saying that Pelosi will not call for a vote unless she knows it will pass are these:
You can't call it up twice. Once defeated it is dead. So if she calls it up to a sure defeat that is the end for ObamaCare 2010.
There is the real risk that post the November elections you will have a lot of lame duck Dems who would have voted NO but will give one for the team on their way out and vote YES and Obamacare will pass sometime in late November or early December. If Pelosi votes now and loses she will not have the opportunity to test that idea out.