The iMac question really is only interesting when discussing wether or not Apple will put LED's in the displays (or at least in the 24", Apple clearly has them now).
The speed bumps can work if Apple decides to offer the 3.06 as the standard processor, but that will leave off any BTO options for processors, which as far as I know hasn't happened in a long time (if ever, I am not sure).
I really would like the option to drop the Superdrive in exchange for a second hard drive. It would be great to have the main drive be a SSD, while a second drive be a large HDD for media. All Apple would have to do is make the USB port compatible with the Macbook Air Superdrive.
As far as Firewire goes, I believe they will consolidate it and leave only the Firewire 800 on the iMac, just like they did for the Macbook Pro. Maybe they will add another USB port in its place which would be nice.
Also, the LED's are thinner and run cooler I believe, so they likely could get a really good graphics card in there as well. The BTO for the 8800 is pretty good, but it would be nice if that was at least the standard card. They cannot do another iMac update and keep the ATI 2600 series in them.
It seems a minor bump is coming, which will require no event.
As far as Macworld goes:
(1) Mac Air (new Mac Mini)
(2) 20" & 30" LED's
(3) iLife '09
(4) iWork '09
(5) Snow Leopard Update (which really isn't big, but we can probably get an idea of a release date and price)
The Mac Pro will likely get an update right before either the holidays or shortly after Macworld. The Nehalem chips for servers are ready by the end of the year, which I believe the Xeon is part of.