it seems as though many companies will be releasing holsters and or belt clips for this phone... you may want to steer clear of the ones that close with a magnet for the reason it could cause the phone to go into GPS or HOME mode.
i can promise you there will be several options from the VZW store at the release.
The G1 has an actual micro compass in it (for Google Street View), which magnets in close proximity tended to screw up, so G1 owners learned to avoid cases with magnet closures (not easy). I don't know if the droid also has a micro compass in it. Anyone know yet?
I plan to buy a droid today. If I decide I want a belt clip, I will be buying a Seidio, once they have them available for the droid. IMHO, they have the best quality (and no magnets are involved).