I think 15" widescreen is kinda obselete for computers now.
My old x41 (IBM all in one) at home has a 15" standard screen and my sister's 13" MB is the same width, and i think a 15" MBP is the same height. Not sure. But anyway, the (new) widescreens that have a larger footprint are really smaller if you don't consider the extra width. So I think 17" is the new 15".
I'm also used to a 20" screen from my high school's graphics lab. Going from the PowerMacs we had to PCs in the building with 15" standard screens was unbearable.
I really think 15" is done (for desktops). But who knows. Maybe with this new resolution-independence it won't really matter anymore like it does today.
-=|Mgkwho
I agree the 15" is pretty much gone from desktops, and there's no reason to bring it back. 17" widescreen is about as small as any new computer should really be used with.
On the topic at hand, though, the next iMac... well, I'm not sure that the iMac G4 would be the backwards step they take - it's a cool design but it was so lauded that they might get some negative press from recreating it. "Apple's run out of ideas and are going back to their best designs" type comments. And, truthfully I could sort of see the logic there.
So, two options that I see:
1) They keep the AIO desktop (which I think is stupid - they sell some fantastic laptops that could fill that market need perfectly). In this scenario, I envision a throwback to the old acrylic displays. An LCD with an L-shaped stand that houses a computer in it. Very similar to the current iMac, but it's much thinner, and the computer bit aren't directly behind the LCD. This would have some advantages from a technical standpoint, such as allowing the optical drive to be horizontal (which helps performance slightly), and spreading out head to a place which you could easily use a lot of passive cooling and minimal fans (which people like).
2) They ditch the AIO, and introduce a third laptop line between the MB and MBP (the iMacBook) which has MBP features, but isn't super thin and light, has less battery life, etc, and is designed for primarily home use with occasional travel, which is how a lot of people use their MBP/PBs anyways (esp the 17" models). They then replace the iMac with a mini Pro design. Something either like a way scaled down Mac Pro (looks wise) or a scaled up mini. Specs similar to what the iMac has, 2 RAM slots, 1 3.5" drive bay, 1 5.25" drive bay, a handful of USB/FW ports, but add in an Express Card slot and a PCI-Express slot for upgradeable video.
If they did #2 they should release a new line of displays, moving the 20" to the consumer line, and leavinG the 23 and 30" for a pro-line. Add a 17" or 19" 1440x900 display to the 20" in a new case that is styled like the current iMac. No FW ports, but keep USB on them. Get the costs down to $200 for the 17" and $400 for the 20". Add component and SVideo inputs the the 23 and 30" models to help distinguish them.