[justjohn]

One other thing I would add is to check on different variations of Android (stock, HTC's Sense, Samsung's TouchWiz, etc..) as well as resolutions, since they have different default widget styles and colors that can affect your app in different ways. (Android 4 is much better about this with Holo, but anything before it can vary a lot.)

[radley]

If you stick with native styles for device elements (such as the action bar & notifications) that should cover 99% of requirements.

Widgets are a different matter. Suffice to say the big difference there is between Samsung/HTC who displays full-sized widgets and Nexus/Motorola/Asus who force widgets to use way too much padding.