Al Queda has been badly mauled by US and allied military (and covert) operations in the past 1 1/2 years, and badly needs to reassert their relevance in the Arab world. They need something tangible to brag about, even if they have to make it up.

Another interesting angle is that as long as Iraqis believe that underground Baathists are responsible for the attacks on allied and Iraqi security forces, the vast majority of Iraqis will prefer the (much) lesser evil of US and UK occupation, no matter how lengthy. Iraqis understandably want to be in control of their own country, and they don't want foreign troops on their soil, but what they fear far more than anything else is the possibility of the bloodthirsty Baathists someday returning to power. That's why most Shiites and Kurds, and even quite a few Arab Sunnis, approve of the occupation as neccesary on the road to reconstruction. That's why Al Queda is trying to take credit for the Baathists attacks - they're trying to discredit the Baathists, and convince the Iraqi people to no longer fear them, hoping that the Iraqi people will then no longer see the need for the US/UK occupation.