I've had the N800 since it was first released, and still use it a lot.
However one thing I can't figure out, it has GPS, car mounting kit and supports bluetooth
to a phone, yet it can't do hands-free calling to another Nokia phone but it can do SIP calls?

I've looked around for ages and not found anything, am I missing it somewhere obvious - or is it just me who thinks this should have been available ages ago?

mmm...
What I was saying is can't I use the N800 as a "bluetooth headset"?