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).
The way to indicate that an application should only run on iPhone is to specify your application as an iPhone type application, rather than universal. Open your project (in XCode), click on the project name at the top of the Project Navigator sidebar, select the target, go to the summary tab, and change "Devices" to iPhone.
When you submit it, it will only be run in emulator mode on iPads, thus getting around any issues.
You can't restrict the app to not work on the iPad. Backwards compatibility with all iPhone apps is a feature of the iPad. Your app will run in a 100% frame or in an optional 2x mode depending on user preference.
iPhone OS apps that link against the 2.x or 3.x framework and test clean on the iPhone and iPod touch should work w/o any trouble on the iPad.
If you tested on the iPod, taking into account the lack of cell radio, camera, etc., you should be totally fine.