Maybe they weren't quite finished with the MacBook Pro? and maybe there weren't enough Intel Core Duo chips to ship both new systems in January? *shrug*
The iMac will have been easier to get out the door because it doesn't have the same power draw worries as a notebook.
The iMac also seems to be using a different, higher voltage variant of the processor than would be used in the MacBook, and a simple matter of availability from Intel could be part of the shipment delay.