If you have installed the iPhone SDk on a Power PC, you can't create iPhone Apps. Because the PowerPC Plattform is not supported by the SDK. Well...., you can install it browsing through the package content in the SDK image disk.
You must intall the aspen packages manually to use it with the XCode. You can se the packages in the image.
After install it you need to move it to the platform folder in the Developer folder. These packages by default will be installed in the root folder, you need to move it to the developer folder. Other option is change the destination folder in the installation app.
After these steps you will see in the new project window the option to develop iPhone apps.