It's not delayed. Watch the Keynote, Steve said that the 3G would not ship till late April, whereas the Wifi would be available late march.
Slightly different manufacturing process, they probably had to add an extra conveyer at the factory branching off on the main wifi line, and a few dozen people to man that conveyer, to add the 3G chip. That takes ramp up time, not to mention training time for employee's to install the new part. It's even possible that the manufacture didn't know about the 3G model until a few hours before the keynote, as we know how much apple loves keeping people in the dark.