Just to be clear about this -- Apple doesn't release new models of Macs on a regular schedule -- ask any Mac Pro user.
And most of the time when they DO release an update, it's a minor one -- slightly faster chip, maybe a larger default HD, that sort of thing.
Laptops TEND to get updated somewhat more often than desktops, because of the huge rate of change in that segment (desktops are becoming less popular, so Apple may well decide it's in no rush to improve what is already a pretty perfected product).
My best GUESS (and that's all it is) is that we might see a SLIGHT bump in the MacBook Pros this fall, and MAYBE a new iMac early next year. Both models have already made the leap to Thunderbolt and Core i7, so they are going to be good to go for quite some time. MacBook Airs, Mac minis, MacBooks and (dare we hope?) Mac Pros are all due for a revision before the year's out, but this may not leave much room for a bumped MBP, especially considering (again) there's not a compelling reason on the horizon for them to make any changes to a hot-selling model.
Bottom line: no revision to the iMac or MBP expected in "the next few weeks." Buy with confidence.
(aside to Bob: MacNN/Electronista is a great site for keeping up with Intel's moves. Behold!
Intel's Ivy Bridge roadmap narrowed down to March 2012 | Electronista)