Here is how I manage to establish a network connection
between N900 and my MacBookPro over Bluetooth PAN.

On N900: Edit /etc/bluetooth/main.conf and remove 'network' from the
configuration option 'DisablePlugins'. Reboot device.
This enables Bluetooth PAN.

Now pair your PC with your N900. On OSX: Go to network
preferences and add interface 'Bluetooth PAN'. Assign an IP
address to this interface, like 10.10.0.2 (Do not use your
LAN subnet like 192.. or 172.. to avoid routing issues).

Now grab your N900 and assign a IP address to the bluetooth
interface:

ifconfig bnep0 up 10.10.0.2

Now you should be able to ping and ssh 10.10.0.2 from your PC.
For ssh: be sure to install OpenSSH server on your device.

Have fun,

euer datenpeter!