I think we all need to get ready for a radically different approach to personal computing in this next revision of the iMac. I suspect that the reason iMacs have been delayed are for these reasons:
1-iPhone and the introduction of new technologies built around OS X (10.5), touch screen and gestures. If you think the iPhone is cool, wait till you see it on the desktop
2-the release of 10.5 in October which will likely include iPhone technologies not demoed at WWDC (again, gestures)
3-iLife is rumored to have gone golden master. Expect major changes to most if not all applications to support gestures
4-Apples latest web site update screams gestures
5-rumors of a gestural mouse and track pads (in future laptop systems)
6-scale the iPhone screen technolgies up and build that into iMacs and Laptops. The next version of these systems will have virtual, but full sized keyboards. Remeber that rumor not too long ago? (Can some one find that post?)
7-Persistant rumors of an iMac case remodel. It is time for the plastic white casing of the iMac to go away (IMO). Look to the iPhone for styling and materials ideas. I think, brushed aluminum back casing, black (or white) inner from casing with built in isight and impact resistant glass touch screen.
8-rumors of 3 finger gestural commands in iPhone OS may hint at two handed controls and other UI innovations.
9-I wouldn't be at all surprised if this next version of the iMac could be made to lay flat or at a slight angle on the desktop.
10-Expect Video iPods to be released, using iPhone technologies, ahead of any iMac. I suspect iPods will still be HD based with capacities of upwards to 160GB.
11-300 plus days and counting since the last iMac Rev. Apple could have easily had an incremental revision in this time. To do so now, in my mind, wouldn't make much sense. Again, leads me to suspect a much bigger product announcement once the iPhone hooplah dies down abit.
I really think that a lot of these rumors are converging to something very different than we might expect. Whether it comes this August or September or at MacWorld in January is open to speculation too. If it is announced and ready to ship this year, and if it ships ahead of Leopard, and if I am right about the screen and gestural support then I would expect they would ship with coupons for free (not including shipping charges) upgrades to 10.5.
This is my 2 cents based on what I've read into the disparate rumors and the connections I've made that seem to converge to something like this.
But then again, it could be a heck of a lot of wishful thinking on my part and It is so darn hard to be patient and wait with the rest of you!