A CNET article points out:
Supplies of Apple Computer's iMacs are dwindling at resellers and distributors, a sign that changes could be in store for the company's venerable all-in-one computer.
and...
Changes may also be afoot for the PowerMac, Apple's professional line. All of the single-processor PowerMacs show a 10-day wait for shipment at Apple's online store. The reseller noted there are still quite a few of the current PowerMac models in the distribution channel but said he expects upgraded models to be announced soon.
(updated: Apple's Keynote will be broadcast/streamed.)