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. Remember 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 front shell 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 a bit.
12-New version of Safari supports adjustable fields for entering information. Again, pinch or 2 finger resizing gestural support?
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!
I disagree completely with your prediction of a multi-touch iMac. The iPhone uses a touch screen in place of a mouse only because it doesn't make sense to plug a mouse into your phone. The iPhone does not use multi-touch because it is better than a mouse. A mouse is much more precise, as the tip of the mouse is much finer than the tip of a finger.
Also, imagine the fingerprints! Go look at a touch-screen kiosk at a mall and think of an iMac with a screen that dirty.
The cost is also quite prohibitive, especially for something so gimicky. Again, it is only on the iPhone because people don't want to hook up a mouse to their phones.
The rumors of the multi-touch mouse are probably along the lines of the current mighty mouse--one that only has one button, but senses hand placement to act as a two-button mouse. In the case of a revised mouse, the surface might remove the scrollball and simply react to a finger sliding across it.
I don't believe the iPhone is now the basis for all future Apple designs. You'll notice that the clickwheel hasn't been transplanted onto any Mac computer just because the device is popular. Clickwheel is to iPod what multi-touch is to iPhone (though iPod will probably get multi-touch at some point). I believe that the iPhone technologies exist for the iPhone alone, and perhaps some future Apple handheld devices (i.e. new iPods). I can't see the logic in dumbing down the computers with a mobile handset-inspired interface or control mechanism. It is like a company building a car that has no steering wheel just because they think it is cool that trains don't have one. It'll drive for a while--it might even be "cool" for a while--but you probably don't wanna take an extended trip in it.
I really like the idea posted earlier about simply calling the new machine "Mac." Simple and sweet. AND it fits in perfectly with all of Apple's commercials that show the iMac at the end with the word "Mac." Many consumers--particularly those not familiar with Apple's offerings are more likely to ask "Do you sell Macs?" rather than "Do you sell iMacs?"
So now my prediction: Specs bumped up to match the Macbook Pro (2.2/2.4, 2 gigs ram, etc), Wireless keyboard/mouse standard, New iLife, support for 4 gigs memory, better iSight, superdrive with a BTO Blu-ray drive, optional firewire hard drive that hooks onto the back (keep the wires to a minimum) of the unit, to be used with Time Machine.
As for the casing, I believe that the machine will use a material similar to that on the current iPod nanos, with those colour options. Some people have said that mixing aluminum/metal finish into the consumer line will be confusing. I believe the all-in-one desktop looks different enough from the Pro tower for most to buy without confusion. I apologize in advance to any pro users who accidentally buy the all-in-one instead of the Mac Pro tower, or those consumers who buy a Mac Pro and keep trying to find the built in screen

. Those same individuals might accidentally buy a head of romaine lettuce instead of a cucumber because the colours are similar. Seriously though. Apple has 3 (if you count the mini, which is metal finish BTW) desktop offerings. I believe customers are capable of differentiating between them. Also, as the Mac Mini is metal finished, a metal-finish iMac (sorry, "MAC") would actually unify the desktop offerings.
Or maybe it will just be white. The chin will be reduced, but not down to the level of the cinema displays.
Also, no coupon for OS X Leopard. That's how they make their money!