Valid points, but I am looking at this from another direction.
Democrats are creatures of big government, of big spending... but above all, they are creatures of self-interest. If the choice is sticking with (what they call) principle or looking out for No. 1, they choose self-interest.
Suppose the GOP were to win a good majority in 2010, based on the public anger to Obama's policies. If the GOP were to stick to principle and pound on cancelling Porkulus, and the public sentiment against it continued to rise to the danger level -- I think that some of the Blue Dogs, fearful of their chances for reelection and facing angry electorates in their home states, would come out and reverse course on Porkulus. I think they would throw Obama under the bus if they thought it would help preserve their own careers.
That is where a veto-proof majority could possibly come from.