@YoDude: I tested it, and it's definitely not the proximity or light sensor, but is magnetic. Like you, I hope someone came up with a workaround for the N810 which can be transferred to the N900, but I wouldn't hold my breath

I was shopping in the store tonight, replacing my nice leather magnetic-clip case with a cheaper-but-okay slip-in case. 95% of the cases in the store were magnetic, so I had to keep searching

I'll probably eventually get a Piel Frama or similar.

Correction: Piel Frama is also magnetic. S**T!!!