When testing on my iPad 3,
[[UIScreen mainScreen] scale] == 1.0
My understanding is that this should be 2.0.
I am using the iOS 5.1 SDK with Xcode 4.2 on Snow Leopard, which I understand is not an "officially supported" configuration. However, I can't imagine why that would affect what is happening purely on the device. (I can imagine all sorts of ways it would break the simulator.)
Before XCode version 4.3, [UIScreen scale] will return 1.0 for an iPad 3. Updating to 4.3 will enable the correct value of 2.0 being returned.