I posted this in another thread:
Out of curiosity I looked at the buyers guide for the amount of time between updates. The shortest time was 71 days for the introduction of the then new 20" iMacs. The next shortest period is up to the introduction of the newest iMacs at 90 days. Apple had good reason to try to push a new Mac out at Macworld and gave us the iMacs (and MPBs). If you discount the short time frame for these two "tweener" updates the average time frame goes from 181 days to 236. BTW the longest time difference was 287 days, 358 if you discount the addition of the 20" iMac as an upgrade (as opposed to a processor change).
Whatever the case may be I could see Apple applying the new upgrade, but probably not until a while yet. Even at 71 days between updates we are looking at late March (3/22), 90 days is early April (4/10). I wouldn't think it likely, but obviously it has happened before. An average time frame would place the upgrade sometime after early July.
I have too much time on my hands, and it's all based on past experience. It has nothing to do with little things like the availability of parts, effects on the market and little things like that. Anyway, I would guess you would not see an upgrade until early summer, maybe for WWDC or soon after for the new school year buying season