It seems pretty likely from what I've read that the people behind this were Islamic terrorists, rather than supporters of Saddam. The kind of people who detested Saddam, and vice-versa, and who are now flocking into Iraq to get into the action.

Here's an extracxt from what a commentator in today's Guardian (Easy targets are magnet for Islamic militants) wrote about this:

"Though nobody had claimed responsibility for the suicide truck bombing last night, the method of attack suggested that the culprits were more likely to be Islamist militants than disgruntled supporters of Saddam Hussein.

"Truck bombs brought al-Qaida to prominence in 1998 with its attacks on the US embassies in Tanzania and Kenya.

"There is also evidence that Iraq has become a magnet for militants from all over the Middle East, either to escape surveillance in their own countries or to engage in jihad against easy targets."