My laptop has wifi, my phone has bluetooth, my n800 has both does this mean I can use the n800 to "bridge" the connection and connect to the net with my laptop using my phone's 3G connection?

Not with any current software. Technically it should be possible by installing some NAT software on the n800 and using that to share the bluetooth connection you have with your phone over wifi and then configuring your laptop to use your n800 as a "router". However, as far as I know, no NAT software has been ported to maemo.

Hm, iptables run as far as I know.. so it would then be a question if the nat module is compiled in, or could be compiled in. I set up my linux laptop to work as a router to the net from my BT PDA, I imagine the N800 could be set up more or less the same way. Possibly some more bluez modules would be needed on the N800 (rtcomm, for example). Another thing is that BT and wi-fi share the same antenna on the N800 (by multiplexing), but this works reasonably well as far as I can see.

Yes there is a NAT module compile for maemo... Is here in the forum just search for iptables.