I think Apple adopting some AMD chips would cause confusion, especially to users who don't know much about computers. Besides, there isn't really a standard way to compare the speeds of Intel and AMD chips so who's to say that an Intel (blah blah blah) is faster than an AMD (blee blee blee)? Like I said, confusing to the normal user.
To us, it probably wouldn't be a big deal because we peruse these sites day-in and day-out, at home, at work, we read up on all the processors and whatnot. But the remaining 80% (at least) of users don't know what-from-what.
Secondly, adopting AMD chips alongside Intel chips would, likely, split the Apple's Pro-users down the middle, some supporting Intel, some supporting AMD.
Thirdly, even though Intel and AMD chips have identical instruction sets, they don't have identical pin-layouts. Apple would, thus, have to create two motherboards for each computer, one for Intel, one for AMD.
Fourthly, Intel and Apple are buddy-buddy right now, since Apple is one of Intel's larger customers nowadays (note the special presentation slot given to Apple) and as long as they remain that way, Apple will reap the benefits thereof. These may or may not include cheaper prices, Apple soft/hardware optimization consideration in future chips, etc.
Fifthly, Apple has written OS X especially to take advantage of multi-core CPUs. (This means nothing in terms of third-party software, but it's at least a start.) Intel is beginning to sprout multi-core chips out of its posterior. A octal-core Mac is theoretically right around the corner (and if Apple doesn't release one, the user can upgrade him/herself). In AMD chips, I don't see anywhere near the progress Intel has made in this area.
Five Point Fively, Software Developers are beginning to write their software to take advantage of multiple cores, giving Intel a further advantage.
Sixthly, I should be working right now, so I'm done.
-Clive