If you are deciding how to layout your UI, that is what the screen size and orientation configuration is for.

I just would not try to define what a "tablet" is. In fact, forget the word even exists. If you look in the market, there are already so many devices out there spreading between phone and big 10" screen that there just isn't any clear line you can draw. And it definitely has nothing to do with a default orientation (in fact apps shouldn't think about default orientations at all).