Courts have long held that Congress cannot "bind" future Congresses—that is, it can't force a future session of Congress to carry on its own policies. That practice, formally known as "legislative entrenchment," is seen as privileging one group of lawmakers over another, "binding" future to the priorities set in the present. In the 1996 case U.S. v. Winstar Corp., Justice David Souter quoted the British jurist William Blackstone, who said that "the legislature, being in truth the sovereign power, is always of equal, always of absolute authority: it acknowledges no superior upon earth, which the prior legislature must have been, if it's [sic] ordinances could bind the present parliament." The principle is more complicated in the United States, where the government is bound by the Constitution and any private contracts into which it enters. But as a general rule, any Congress can reverse the decisions of any past Congress. For example, Bob Dole repealed future tax cuts in the 1980s.
Our political system makes promise-keeping even harder. Congress always has the incentive to act in its own short-term interest at the expense of the country's long-term interest, since spending is more likely to get you re-elected than saving. The result is that Congress always puts off hard financial decisions. (Article at Slate).
(This clip shows savings of 1 billion in year 1 and supposed 153 billion savings in year ten)
The only real savings are ones that kick in before the 2012 election.
Added: Good story (that also includes Wimpy) from Roger Kimball who calls the cuts phony, the taxes real, and introduces the term, "mendacious equivocations".