Back in the day the main benefits to the MacPro were the expansion slots, dual processor, extra hard drives, and dual processors.
Now days with iMacs coming with quad cores, 16 GB of ram, and terrabytes of hard drive space, and thunderbolts ability to add external storage, and an expansion slot chassis; I think this is an obvious move. Add a duel processor option to the iMac and there you go. The only people this will hurt is the people that use Mac OSX Server as the MacPro and MacMini is the only server hardware they currently offer.
I agree iMacs are pretty capable machine nowadays for much work that formerly would have required a Mac Pro. But one problem with this approach is the built-in monitor. The iMac's monitor is "good enough" for many, and it's not really bad, but:
a) it's not pro quality, black levels and hence contrast is poor and there's a lot of edge backlight leak, color gamut and accuracy is only so-so.
b) it's built-in and cannot be changed or upgraded.
I wonder if there will be an "iMac Pro" model that would use the Thunderbolt/Cinema display, as well as offer more pro-level options like 6 or 8 CPU cores, dual drive slots, faster GPUs, more RAM slots, etc, but in an iMac enclosure. That's still a step down from a MacPro, but it might cover a lot more people who need that stuff, and still allow some manufacturing cost savings by using an iMac casing and other components. But I still think it makes more sense to have a monitor-less solution, maybe based on a Mini chassis instead of an iMac, as iPedro suggests it could be a modular system.
