A MacRumors post suggests that the recently released, "Intel-only" iPhone SDK works on PowerPC Macs as well. According to the post, it worked fine on a iBook G4 running Mac OS X Leopard. The post did go on to say that Xcode displayed an error message upon trying to build a project, saying that the "target architecture does not match." Errors like these are to be expected, but at least those PowerPC-using developers can start to develop for the iPhone. There is no word yet on how the $99 digital certificate will work on the PowerPC Macs.
3by9's website has the full details on how to get the SDK running on your PPC Mac.
[original post by 3by9]