Actually, AMD has some good stuff. I'm not an x86 fan by any means, but I'm quite happy with recent Opteron boxes running Solaris or Gnu/Linux. They are amazingly quick and smack the poo-poo out of the Intel boxes on FP.
However, I can understand why Apple (having chosen x86) went with Intel. Intel is more focused in the arena Apple likes: portables and consumer desktops. I'm more of a server sort (The E5000 mentioned below in in my house, so I don't really care as much about the desktop arena.
Now, an Xserve with dual-core Opterons would make me tingle all over. But I don't see it happening, as too many companies have similar systems out already, and it would be much tougher for Apple to play in that field.