Hi,
more than year later, I have the same question. I've started to play things as described elswhere on my debian squeeze, but soon I stucked on n900.

on root@n900:_________

/etc/bluetooth/main.conf and removed 'network,input,hal' from the
configuration option 'DisablePlugins'.

into /etc/network/interfaces I added

auto bnep0
iface bnup0 inet static
address 10.10.0.2
netmask 255.255.255.0
gateway 10.10.0.1

(even after reboot) I ran

ifup -f bnep0 (on ifup -v bnep0 it says 'interface already configured')
ifconfig bnep0 up 10.10.0.2
...ifconfig: SIOCGIFFLAGS: No such device

sabhack - did I forgot something?
thanks, j.

---edit----------------
ok, one thing is clear. Only after I try to connect over BT with nm-applet to "mymaemoname"_Network, the interfaces become available on both sides. No problem to add few lines to local /etc/network/interfaces (like
allow-hotplug bnep0
iface bnep0 inet static
address 10.10.0.3
netmask 255.255.255.0
# gateway 10.10.0.2
)

I must launch the ifconfig manually on n900.
However - I can have a bnep0 connection for about 30 seconds and after I get disconnected.... nm-applet just rotates ... these 30sec. This I dont know where it comes from, it doesnt happen for debian->BT->n900->mobile broadband.

Next thing: to be able to run dbus-scripts, I realized that it must be installed (from -devel?). I am not going to go that far, I am quite happy with the stability now. Are there any other solutions to respond automatically on n900 side?

-----next edit ------------- sorry ---------------------
after a lot of google ......
nm-applet 0.8, although it says it is ok, it has an issue with PAN
Finaly, I removed my bluetooth-applet, installed "blueman" and used blueman-applet. It didnt work neither, BUT, right-click/local services, I ticked group network, for "pan support" I tried to tick blueman (maybe a bug..., doesnt remember the setting) AND right-click/plugins I checked "DhcpClient" which unchecked "NMPANSupport". Than, "group network" from blueman started to work, nm-applet remained untouched and unaware.
My previous DUN setting in nm-applet remained ok (no idea where it is stored).
------------------
The only question remains, howto set
ifconfig bnep0 up 10.10.0.2
on n900 when PAN is started... without loading stuff from extra-devel.....
thanks, if there is any advice