I guess you are also maintaining a data connection all the time while Bluetooth is established... what kind of data connection do you have on your plan? a 3G call/connection may use power more quickly than a 2G GSM/GPRS connection (unless 3G has some power saving tricks up it's sleeve) in which case it's probably not the Bluetooth connection that's draining the battery but the phone-to-carrier connection (which is a by product of the Bluetooth connection).

How long would your phone battery last if you were to establish a voice call and not hang up? 3-4 hours?

To take the phone-to-carrier connection out of the loop, establish an OBEX (file transfer) Bluetooth connection between your N800 and phone and see if you observe the same rapid power drain on the phone.