Moving to, or simply including a 'budget' line of AMD powered Macs wouldn't be a big deal at all.
The vast majority of everyday computer users don't know the difference between AMD and Intel, anyway.
AMD is more than capable of meeting Apple demand, by the way, considering that if Apple were to include an AMD option, that option would likely only represent a portion of an already small market share.. and more than likely in a low end 'budget' machine.
What Apple has learned over the years, is it's best not to box yourself in with a single part manufacturer like they did with the PPC. Their migration from ADB to USB, from NuBus to PCI & AGP.. Apple has really been making the transition from proprietary hardware for some time.. the actual CPU was really the last piece in a much larger puzzle.
As mentioned earlier, many people in the 'osx86' camp have successfully installed OSX on AMD powered machines, and in many cases, with great results rivaling that of the higher end Intel powered machines. The only stumbling block appears to be that Apple has been using specific Intel motherboard chipsets, which aren't overly AMD friendly.
It would be easy for Apple to include AMD processor support in 10.5, and release it along with a line of sub $500 iMac machines.
Although I suspect Apple probably enjoys a nice price break on the Intel hardware, a price break that very well could hinge on Apple being an 'exclusive' Intel customer.
I personally have never cared for AMD processors much.