In this article titled "Supporting Tablets and Handsets" they explain fundamental approach to Android design. In future, designing just for phones or just for tablets is not an option. All apps should be designed and built from the beginning to scale to all screens.
So I urge you to head to this URL http://developer.android.com/guide/practices/tablets-and-handsets.html and take few minutes to read it though carefully. It is time well spent. I promise.