Second, while Apple's mobile processors are currently the best in ARM world, for "pro" desktop computers Apple needs desktop-grade processor. Making something comparable to i7 isn't such an easy and cheap job.
Besides, there's no Carbon to Cocoa or CodeWarrior to Xcode port, so aside from a very small percentage of optimized assembly code, it shouldn't be nearly as hard.
Good point, I guess Apple never though of that.
I though one of the major reasons for the switch to Intel was the power consumption and performance on laptops.
Qualcomm, NVIDIA, and Cavium have created very powerful ARM chips for non-mobile application. Apple definitely has the chip architects, the money, and the experience to do such a thing.
(Amd, Intel, and now Apple)
By the way, don't forget Broadcom, Marvell, Cavium, IBM etc. as they are doing big bucks with CPU's and SoC's as well.
I'm specifically worried about the performance workstation/gaming cpu market, which Broadcom Marvell, Cavium, and IBM are not major players in. It's currently AMD and Intel. Apple would need to sell a lot of macs with the new CPUs to make the R&D cost worth it and the new CPUs would have to be very fast or they won't sell macs.