Since the Haswell processor will be released in April and it is not compatible with Ivy Bridge motherboards, then maybe they will wait until April for the iMac. There are heat issues with Ivy Bridge and that may be a reason why to keep it out of the iMac.
So Apple would forgo tens of millions of dollars in sales over 6 months to avoid the inconvenience of modifying the board at that time?
There's a LOT of different theories on why Apple would not release a new iMac among other new product releases. BUT, with every minute that goes by Apple is losing sales. IF (a big IF) the new iMac is ready and available, Apple will release it.