I have just uploaded my first app to iTunes Connect and noticed that my list of supported devices is appearing as follows...
Device Requirements: Compatible with iPho
I don't see a good reason to exclude iPad since iPhone apps will run in emulated mode in iPad after all. It's the same situation as a 3.x firmware running apps compiled from 2.x SDK.
To restrict at device level, you add the UIDeviceFamily key, but this doesn't support excluding iPad (just excluding iPhone).