In windows I've used gpsgate software which is a gps-multiplexer much like gpsd, but it has one nice feature which should and could be implemented also with gpsd.

Gpsgate sets up serial bluetooth service and acts as a real bt-gps. I've used it many times with my cellular phone and even with my 770 and Maemo Mapper. The only difference between gpsgate's bt-gps emulation and real bt-gps is that emulation supports multiple bt-connections at a same time.

Unfortunately I do not know bt-services enough to implement this myself.

I'm not sure I understand the details.. is gpsgate an application you execute on a device with built-in GPS, and, in addition to being able to serve multiple on-device GPS applications it can also sets up a BT interface so that other, external devices (e.g. 770) can connect to your GPS/windows/gpsgate devices, just like to a stand-alone BT GPS? Is this how it works? If so, it sounds useful.. it could be put directly to work on an N810 so that those with both an N810 and a 770 or N800 won't have to carry a separate GPS for the 770/N800
(or maybe the N810 can look as a BT GPS already, for all I know).

Or is it something different?

It works like you described. Incoming gps can be wired, internal or bluetooth and it provides virtual serials for local apps, bt service for other devices nearby and can send location over udp for remote tracking.

I've used it for remote livetracking which shows my live position on googlemaps. I've also done tiny udp-sender using gpsd as location source.With gps-emulation I could stop using gpsgate.

After heavy google usage, I've found that it has been already done:

http://www.xmission.com/~bmidgley/gumstix/

Will try it real soon now!