I have Mac OS 10.6, XCode 3.2, and the iPhone SDK for Snow Leopard installed. When I try to build to my phone it fails, because there isn’t a phone attached with a supported OS. Thats because XCode supports iPhone OS 3.1, which isn’t out yet, and iPhone OS 3.0. My version, the current one of 3.0.1, can’t be built to!
I think I’ll wait for 3.1, rather than downgrade to 3.0 or download the 3.1 beta. Strange though that 3.0.1 isn’t good enough for the 3.0 builds.
Go into your…
MacHD → Developer/Platforms/iPhoneOS.platform/DeviceSupport
Duplicate your “3.0” folder and name it “3.0.1”
Fixed 
I just installed Snow leopard and it works just fine when building on a 3.0 device. You probably just need to re-copy your provisioning profiles to XCode.
Or follow the one liner apple created, which basically does what JTBentley says but without copying. The magic thing is creating a link through the ln terminal command
You should be able to find the corrected command line (the apple docs contain a wrong one, didn’t work for me and I have it installed in the default location but my 3.0 folder has no (xxxxx) at its back) through google pretty easily. took me 3 mins
Thanks, that got me further. This would be the first time I’m building an app onto my 3GS, and it seems reluctant to take my provisioning file. If I try adding it, only one of the provisioning files is enabled in the file list, the others, including my valid one, are grayed out.
I’ll figure it out, I’m sure!