I can't see Apple switching Macs back to PPC any time soon. In the last 10 years they've forced Mac developers and customers to switch to a totally different operating system AND processor architecture. Both were good moves, but it seems silly to switch back.
Apple has said they bought P.A. Semi to create custom chips for their portable devices:
http://blogs.wsj.com/biztech/2008/04/24/jobs-still-hearts-in...