In WP 7.0 it was easy to test the tombstone system - I'd hit the camera button then press back to get a tombstone/resume. In WP 7.5 the app goes dormant for awhile, but appears to tombstone at an indeterminate time. Is it possible to force this situation for testing?
You can test tombstoning using the emulator (this may even work on the phone I haven't tested it). Under the project properties, on the "Debug" panel, there is a checkbox named "Tombstone upon deactivation while debugging".
This setting will force the app to tombstone in the way it did before when the debugger is attached.