You must re-install Xcode using the version on the Snow Leopard DVD. The iPhone SDK must be re-installed afterwards.
Edit: I just noticed that Apple started distributing Xcode 3.2 with the iPhone 3.1 SDK. I would recommend you to attempt a full uninstall of the developer tools, as described in the SDK release notes (see developer.apple.com), and then to re-install it. You should also check that each computer have a unique name set in system preferences>>sharing. As a last resort, I'd check that Snow Leopard was booted in 32-bit mode (it does by default), and (just for the test) try to boot Snow Leopard in 64-bit mode (hold the 6 and 4 keys while booting).
Edit 2: Check this thread, it seems related. Maybe you should try to enable Xgrid in sharing preferences?