It is possible to link a Nokia E61 to a Mac/Windows or Linux box using PPP over Bluetooth. There are lots of tutorials on the net, however I could not get any of them to work, So this is the procedure I used.
NOTE: I had problems accessing the main GnuBox site, but after a quick google I managed to find the gnubox.zip file and did a self-sign using https://www.symbiansigned.com/app/page online signing system.
I am peering the Nokia E61i to a Mac OSX 10.4.11. I found that I could use the standard Bluetooth-PDA-Sync Bluetooth serial device.
The following command starts PPP in no-detach mode and debug enabled so you can see it working. It assigns the Nokia E61 the IP address 192.168.0.2 and uses 192.168.0.1 for its end.
sudo /usr/sbin/pppd /dev/tty.Bluetooth-PDA-Sync 57600 noauth nodetach debug local passive proxyarp asyncmap 0 192.168.0.1:192.168.0.2
And the PPP link should now be running.