Am I the only one who thinks that the Republicans--with the exception of squishes in blue states--do not in fact wish the Democrats to compromise with them? The upside of cooperation is limited--they avoid punishment for having voted against it. The upside of voting against whatever the Democrats put up is clear: they get to hammer the Democrats with it.
To me, Obama's much derided willingness to cooperate with conservatives looks less like either personal magnanimity or weakness than a simple political calculation: if the stimulus does not work, he and his party will lose the White House, and very possibly Congress, in the next election cycle. People are mad at Bush for Iraq, but Iraq isn't going so badly, and the recession is right here, up in our faces. The Democrats promised that Republican ideas got us into this, and that it therefore follows as night to day that things which Republicans dislike, like massive government spending, can get us out. If things look grim come 2010--and if Ken Rogoff's two year rule of thumb for the post-financial crisis decline holds, they will--Americans will vote accordingly.
The extent to which Obama is willing to push concessions to the Republicans measures the extent of his confidence in the power of the stimulus--and I'd say that judging by results, he's at best middling confident. Which is not surprising, considering how many very smart advisors he's surrounded himself with. I'm sure they've explained that the models are only very rough guides at best.