The readme for the iPhone SDK says it is only compatible with Intel based macs running OS X > 10.5.2. Apparently it is possible to get it to run on PowerPC based Macs:
By default, the iPhone SDK package available free from Apple’s site will run on a PowerPC-based Mac but omit the iPhone SDK-related files, installing only Xcode 3.1 beta and a series of other files. This routine is easily circumvented via the shareware tool Pacifist, which can be used to force an installation of the entire SDK package on PowerPC-based Macs.
Follow these steps:
- Drag the iPhone SDK package onto the Pacifist icon
- Select the top entry, “Contents of iPhone SDK”
- Click the “Install” button in the upper-left corner and enter your administrator credentials
[Via iPhone Atlas]