Back in the good ol' Windows Mobile day (CE based) one good get a rough estimate of battery life using the CE CoreDll.dll. Well sometimes at least depending on your hardware. This was exposed via GetSystemPowerStatusEx.
WP7 being SilverLight based I'm assuming these sorts of Interop approaches will no longer work. Is there an API that exposes power status info? I've done a quick search and browse but didn't see anything.