WSJ.com: Trends in presidential job approval ratings
Presidents nearly always start with high approval ratings, but usually they begin to fall with divisive policy positions, wars, recessions, or scandals.
ps: I think President Obama understands the importance of these polls. High approval ratings from the public give presidents credibility with Congress. If he can keep his ratings high, he can make Congress follow his lead. But if his approval ratings fall below 50%, Nancy Pelosi, Harry Reid, Dick Durbin, and other members of Congress will make a move. If the President stays close to the center, he may be able to avoid the post-inauguration drop, and keep the political capital he needs to run the show.