While Apple's formal requirements for the iPhone SDK is an Intel Mac running Mac OS 10.5.2 or later, one enterprising individual managed to get the environment working more or less on his PowerPC iBook.
Image courtesy of 3by9.com
The only oddity Ive encountered so far is that you get 1 Warning when you build, because the target architecture doesnt match (PPC vs. i386) but thats to be expected. Also I dont know if the certificates that Apple hands out once youve paid your $99 fee will work perfectly with this system, so for now Id simply use it as a development environment and plan to get an Intel box running with the SDK sometime between now and Junes AppStore launch. It should be noted that Ive got Leopard running on the iBook. But, it works!