but would you believe it if you were in IMC and it said you were just about to fly into the ground or an aerial
I would think if faced with that situation RR the most prudent action would be to climb immediately and worry about whether the GPS was right/wrong once you've got to a safe height.
I quite agree that a GPS with a terrain database is a potential aid to avoid CFIT. As Stuart Forrest pointed out earlier, it could have saved those pour souls killed through an incorrect NDB approach.
However, I still dont feel that the inclusion or exclusion of obstacles on a database would make any difference to the safety or usefulness of the device. A simple terrain-only database should help you ensure your MSA was correct.