The LD-3W bluetooth GPS is listed under accessories for the N810.

Is this a mistake, or could you really have two GPS devices available?

It's a shame the range of bluetooth is so limited. Differential GPS is very accurate if the data is processed properly. With two GPSs, the n810 could be an electronic tape measure or a device to help you find your car in a car park. How much of a killer app would that be?