N900 as GPRS/UMTS/3G bluetooth dial-up for a Ubuntu Linux PC

If you want the N900 to act as a mere modem for connecting to the Internet via a GPRS/UMTS/HSPA/3G/3.5G/... network from your laptop running Ubuntu Linux v9.10, here's a very brief description how to do that. It's aimed at people who don't want to open a shell and enter something like "sudo pon gprs", but who rather prefer to use their mouse (that's actually a rather ununixy/unlinuxy way, I know):

Preparation

1. Prepare your N900: Install the software package bluetooth-dun from extras-testing.
2. Prepare your Ubuntu laptop:
   1. Install the bluetooth manager blueman (the Gnome default bluetooth applet will not do!)
   2. Go to the Network Manager and create a new mobile broadband connection. (In case you already have created one for a UMTS/3G stick and want to use the same provider with your N900, you don't need to do this.)
3. If you have not done so, initiate a bluetooth pairing between your N900 and your laptop so that you don't have to enter PINs, confirm connection requests, etc.

Using it

1. Activate Bluetooth on your N900 if you haven't done yet.
2. On your laptop, click on the blue Bluetooth "B" icon which blueman puts into the system tray (usually in the upper left system menu bar at the top of your desktop). A list with paired bluetooth devices appears; your N900 should be listed there, too. If not, you need to pair it with your laptop first (as I told you step 3 above).
3. Do a right-hand click on the entry representing N900. A pop-up menu appears.
4. In that popup menu, select "serial connection" and there "dial-up networking". Now you will see three bars indicating signal strenghts and signal quality for the bluetooth connection. More importantly however, your bluetooth phone has now been made visible to the network manager.
5. Still on your laptop, do a left-click on the icon of the Network Manager (system tray). Your broadband connection should appear in the "available" section of the menu, with the label that you gave your broadband connection when you created it (step 2.2 in the preparations) Click it to connect.
7. After a couple of seconds, you now can use the net! Don't be confused by the fact that the N900 will not reveal to you that it just went online via GPRS/UMTS in its system tray (there will be no data connection item appearing near the battery level indicator; only the Bluetooth icon will shine blue instead of white once you activated the serial connection in Blueman).
8. To close the connection, you click again on the Network Manager icon in the system tray and disconnect via the menu.

Bear in mind that the Bluetooth connection is a bottleneck: Bluetooth v2.1+EDR only yields a speed of 2–3 Mbit/s, whereas the N900's HSPA chip can attain a throughput of up to 10 Mbit/s for download (HSDPA) and 2 Mbit/s for upload (HSUPA). If you need a higher speed, you thus might want to try JoikuSpot which connects your N900 to your laptop via WLAN instead of Bluetooth (bear in mind, however, that it offers only a highly insecure connection, since the WLAN only uses the terribly outdated WEP encryption, which can be cracked within seconds).