Check out this graph. No Prez who went on to be re-elected posted Gallup approval numbers under 50% in the re-election year. Bush is at 50% now and hasn't shown any real upward movement since attacking Iraq.
http://www.pollkatz.homestead.com/files/gallupcharts1.gif
Here's a graph with all Presidents since and including Carter, for comparison. Both Carter and Bush I took a real dive early in their re-election years, and hopefully Bush II will follow the same path.
http://www.pollkatz.homestead.com/files/gallupcharts2.gif