I think three factors are at work here:
1) The emergence of a Progressive Caucus in the House as powerful as the Blue Dogs, and as willing to fight for Public Option. Until now, the conventional wisdom was that the Progressives would support any healthcare reform bill as an improvement over status quo and therefore the ones to negotiate with were the Blue Dogs. But the very public insistence by a block of 60 members of Congress that a bill without a public option is dead in the water changes the political calculus enormously. The Progressives are standing up for the same reason the Blue Dogs do -- they come from districts where killing healthcare reform on the principle of a public plan will not hurt their re-election chances.
This throws the blame for "killing" healthcare back on the Blue Dogs if they say no to a public option. If Obama is smart, he would use his "star power" on the voters in these districts to push the Blue Dogs. We will see.
2) Obama discovered he needed the Progressives after all, and they have drawn a line in the sand. Obama and the folks at the center have been surprised not merely by the vehemence of support for public option, but on the way it has become a line in the sand for progressive support of Obama and the Democratic Party. Obama's confidence that he could sooth his followers as he has in the past appears to have been unjustified. MoveOn, SEIU and other organizations that have been supportive, while not yet attacking Obama, have mobilized to save the public plan. Whatever Rahm Emmanuel may think, folks like David Axelrod are aware that they cannot win in 2010 or 2012 without the core supporters willing to make small contributions and mobilize.
3) Ditching Public Option Didn't Buy Anything -- In Fact It Made Matters Worse. Republicans, scenting blood, may have acted too soon in moving into the next stage on healthcare co-ops. The attack by prominent Rs like Senator Kyle and conservative pundits that COOPs were a "Trojan horse" for public option demonstrated two things. First, that COOPs will not attract bipartisan support. They buy you nothing.
Second, the attack on Health COOPs mirrors the attack on public option and demonstrates the futility of trying to bargain. Public option was supposedly bad because it was a "government take over of the healthcare system." Now COOPs are bad because they are really a "public option" which is really a "takeover of the government." And, becuase you gave up public option, it confirms for those still on the fence that public option must have been bad.
It remains to be seen how the situation falls out. What had been the trump card against the old Liberal establishment -- that everyone must fall in line and concede to the "centrists" to win elections and make incremental progress -- is not working. But it is unclear if the Blue Dogs and the DLC are prepared to concede.