Thank you very much for your comment, mfresh, particularly the bit about the cover needing to be on. I wondered to myself "ok, so the cover has to be on...how does it know the cover is on?" Turn out, Nokia used a tiny but powerful magnet on the n800, similar to how they sensed the presence of the hard cover on the 770. The magnet must have been attached with some pretty weak adhesive on my unit....because it is no longer there. I confirmed my magnetic sensor theory using a big powerful magnet from dead hard disk. If I waved the magnet over the rear of the circuit board, I instantly got a "Memory Card Detected" message on the screen. The hard disk magnet was obviously much to big for a permanent solution, so I tried a fridge magnet. The fridge magnet was much too weak. I thought to myself "now where am I going to get a small, powerful magnet at 4 in the morning..." I ended up cannibalizing a set of ear buds and pulling a tiny magnetic disk from those. I confirmed it is strong enough to activate the sensor. With a little work from some pliers, I managed to make a shard small enough to fit in the little square pit on the removable case section (where the original one obviously had been). To prevent future magnetic misadventures, I used epoxy to seal it in place.

If anyone else is having problems with the internal memory card not being detected, I would check to make sure your magnet hasn't gone missing. Thanks Nokia for the robust hardware design!