The US says they have detected evidence that Iraq has mobile biological and chemical weapons labs. When UN arms inspectors were in Iraq, US satellites or recon aircraft were able to catch the Iraqis moving equipment out of weapons research facilities before the inspectors showed up. The Iraqis for quite good at this shuffle, and this may have given them the idea to make some of the labs mobile. 

US officials say they will probably move Central Command headquarters (which controls all US forces in the Middle East and Central Asia) to the Persian Gulf soon.