Industry watchers had expected Apple's Phil Schiller to unveil an updated iMac during his keynote at Macworld Expo. But it was a no-show. Why?

Fortune reports that in a report to clients issued yesterday, Kaufman Bros' analyst Shaw Wu claims it will be out before March, or June at the latest.

Here are his three reasons that the iMac refresh is late.

Apple iMac 2008

1. Apple is still deciding between powering the iMac with either Intel quad-core processors or newer dual-core processors with larger caches.

"While quad-core would provide a material improvement in performance and potentially jump start sales," says Wu, "it could cannibalize the Mac Pro, its high-end tower."

2. The iMac's vents and cooling systems may need to be redesigned, Wu says, "to deal with higher heat dissipation."

3. Apple is waiting for the release of Mac OS X 10.6 Snow Leopard.

"While Leopard would take advantage of multiple cores, Snow Leopard takes it to the next level with better support for multi-core, multi-processors, and OpenCL, with enhanced graphics capability," says Wu.