The rotation of power is the finest political instrument ever invented for the consolidation of what were once radical and deeply divisive policies. The classic example is the New Deal. Republicans railed against it for 20 years. Then Dwight Eisenhower came to power, wisely left it intact, and no serious leader since has called for its repeal.
Similarly, Bill Clinton consolidated Reaganism, just as Tony Blair consolidated Thatcherism. In both cases, moderate leaders brought their center-left party to accept their predecessors' highly successful conservative reforms.
A similar consolidation has happened with many of the Bush anti-terror policies. In opposition, the Democrats decried warrantless wiretaps, rendition and detention without trial. But now that they are charged with protecting us from the bad guys, they've come to view these as indispensable national security measures.
If health care reform passes, there is a pretty good chance that Republicans will eventually be forced to accept it (as they have with Medicare, Social Security, etc.). They know it and Obama knows it. They might be able to overturn cap and trade if it passed (which doesn't look likely), because that would be designed to raise prices on energy, at least in the short term, to help heal the planet in the long run. Deliberately raising people's electricity bills might create momentum to overturn that legislation, but health care is likely different.
Health care reform is designed to be a social welfare program for the poor and the middle class. If history is any guide (and it usually is), not only be this health care reform be difficult to overturn, it will be nearly impossible to cut once it gets going (or even slow the growth of). That gives opponents a very small window to convince voters that it needs to be overturned, but Obama will be there until at least 2013 to veto any changes.
Elements of the new health care system could be changed (or made more 'efficient'), but the basic infrastructure would probably be left in place by Republicans, just as making major changes to national security programs is nearly impossible for a Democratic president. Those are the stakes, and everyone involved knows it.