Just a little prediction...
People who are saying that the Mini cant have 5 usb ports because of the current iMac, I think that both new ones could. Besides, you cant compare a new model to a model that hasn't been updated in a year.
about firewire, I'd say:
next gen Mini: no firewire
next gen iMac and Pro: firewire
about looks:
next gen Mini: thinner, maybe a little wider and black and aluminum.
next gen iMac: no chin, back will be silver
next gen Pro: front mesh will be black, like Tallest's thread.
about specs for the mini:
2.0 or 2.4 Intel core 2
2 gigs standard, 4 gigs build to order
160 gb drive on all models
about specs for the iMac:
2.66, 2.8, 3.06, 3.2
2 gigs for 20 incher, 4 gigs for 24 incher
20 and 24 (obviously)
5 usb ports
250 gb, 320 gb, 500 gb (the same) except high end 24 might have 1tb.
about specs for the Pro:
er...dont really know much about this one.

Check out Tallest's thread
I know this is for the Mini, but I just wanted to comapre it to what I think is coming on the next gen desktops.
Edit: I was thinking, why wouldn't the Mini have DVI/VGA? Most current PC users dont have the newest monitors...but I guess they could just get an adapter.