Ezra Klein explains why Harry Reid is reluctant to publicly give the public option his full support:
Caucus politics present another dilemma: The public option died due to the opposition of Nelson, Landrieu, Lincoln, Lieberman and a handful of other conservative -- and vulnerable -- Democrats. Reid cut a deal with them, and they signed onto the final product. For many, that was a big political risk. The price was letting them say they killed the public option. Bringing it back to the bill will mean they voted for a bill that ended up including something they'd promised their constituents they'd killed. Cross them on this and you've lost their trust -- and thus their votes -- in the future.
My guess is that this is a much bigger deal than a lot of people think. Centrist Dems may be wrong to feel that the public option is a threat, but nonetheless they feel like they made a deal. For good or ill, no political leader can afford to renege on something like that. Not if he wants to stay leader, anyway.