Despite the iPhone SDK saying that it will only work on i386-based machines, it is possible to develop for the iPhone on a PowerMac!

Kudos to Tom Bradford for this solution:
http://www.tbradford.org/2008/03/iphone-sdk-beta-2-possible-ppc-fix.html

Notes: When trying to run a sample app on an actual iPhone device, I also had to install the DeveloperDiskImage package. And it looks like I’ll need to join the $99 developer program to be able to run something on the device.