http://www.iphoneatlas.com/2008/03/10/surprise-iphone-sdk-also-works-on-powerpc-macs/

Though Apple states that the iPhone software development kit requires an “Intel processor-based Mac running Mac OS X Leopard,” developers have found that — with a little leg-work — the SDK also runs 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

The “Aspen” Simulator, which is a Universal Binary, will properly launch and emulate iPhone applications on PowerPC-based Macs.

Windows users are still out of luck when it comes to the official SDK, though developers are currently seeking alternative, Windows-based deployment methods.