It annoys me that Apple are specifically forcing people to use an Intel mac for the purposes of iPhone development, even though there's no good reason why they need to. All of the iPhone development tools are universal binaries, and apart from a couple of iPhone apps (contacts etc.) the whole thing works quite well.
To make it easier, I've put together a iPhonePPCEnabler which can automatically install the iPhone SDK (provided that you've downloaded and mounted the iPhone SDK first) onto a PPC mac. I've used this to develop and test iPhone applications on my PowerPC G5, though it should work for G4s as well.
The installer not only installs the SDK, but it also patches the architectures spec file (needed so that Xcode knows it can build on that platform) as well as modifying the templates to install the application. For some reason, the install process is broken on the PPC, so instead the template created installs a symlink from the target to the simulator's
Applications directory to your target. This may not be desirable in automated builds or other projects.
If you want to uninstall this, delete the
/Developer directory and re-install from the SDK. Use at your own risk etc.
Note: this does not contain any copyrighted information - all of the installs are done on the premise that you've downloaded the iPhone SDK from Apple and have mounted it at
/Volumes/iPhone SDK, which is where this installer takes its packages from.