Well, no more from Texrat, so I guess he expired.

Those pads (on an N800) are USB...
A & B are data (sorry, not sure which was +)
C is ground
D is +5V

Of course, as you can see from the picture I linked, they're laid out differently on the N810:
(But the identification of signals<->pads, or even that this is the same connector, is pure speculation...) I'm betting that's USB, too.

Could Nokia have been designing ahead for peripheral back-plates here? I think not; else there'd be an OTG sense pad, too. I think they just have a fixture you can drop a bare N800 in and provide power and a USB connection for flashing.

Any use of this will probably require software switching or hardware hackery (adding a switch to toggle OTG, and also disconnect the data lines, would permit connection to a PC through the mini B port).