How to force iOS to install iPhone Apps?
If an app is "Universal," then it will contain code for both the iPhone and the iPad.  When launched, iOS detects this, and the app will automatically display a UI that is appropriate for that device.

The only way to change this behavior would be to modify the core system behavior, which could be done only on a jailbroken phone.

The exception is found in applications where the developer has two completely separate applications for iPhone and iPad.  In that case, you could force the iPhone app to be installed on an iPad.