I don't see a 30" iMac on the horizon. I mean, in the last year, they just discontinued the 30" screen, released a 27" one and also released their smallest laptop (11" MBA).
With the success of Mobile devices like iPad and iPhone driving a huge percentage (~80% now I think) of Apple's revenue, I just don't see the fit in their product lineup for something bigger than a 27" iMac.
Have you used the 27" iMac, OP? I'd venture to say it's plenty big enough and almost has the same # of pixels as a 30" display. And if somehow it's too small, there's always hooking up another 27" Cinema* beside it.
*(Or a refurb Core2 27" iMac, that's almost as cheap as the 27" Cinema - or at least it will be after the iMac refresh next week. It's like getting the computer for free!)
__________________
Steve Jobs re: iTunes on Windows: "It's like giving a glass of ice water to somebody in hell"
WARNING: If you see links to ads in the above post, blame the cheesy ad-linking software used by the owners of this website. I do not endorse these ad links. Don't click on them.
The first time I saw a 30" screen I thought it was huge and silly. I now have one and found that you get used to it and it doesn't seem so silly.
The 30" screens have been a 16:10 ratio instead of a 16:9 of the 27". So you gain a few more pixels vertically. That was something I wanted.
I'll agree that the 16:10 ratio is nice and I really appreciate it with my 24" iMac (1920 X 1200) and it helps accommodate the Dock if kept at the bottom as I do and even if it's kept quite small.
But if someone gave me a 30" 16:10 ratio display, I don't think I'd complain — except for maybe the neck strain. ;-)
I much prefer dual 1920 * 1200/1080 vs a single 27" monster but I can see the appeal of the 27" esp in dorm rooms etc where it functions as a TV as well etc.