I wrote something similar to this some time ago, but it was with my old PowerBook and it was running Mac OS 10.4 (Tiger).
I never actually took the time to make my Motorola L6 work as a Bluetooth Modem in 10.5 (Leopard), but, now that I've finished upgrading to 10.6 (Snow Leopard), I decided to have a stab at it again. It took a while, but the result was very satisfactory.
The problem with my previous post is that T-Mobile has probably disabled the telephone number that the mobile should dial-up to. However, this came as a blessing in disguise. It never occurred to me that both the Motorola L6 and the T-Mobile's network in the UK are GPRS-capable, which is faster and cheaper than straight-up dial-up.
In any case, to use your Motorola L6 as a GPRS Bluetooth Modem in Snow Leopard, start the process of setting a Bluetooth Device, which is pretty straightforward.
In the step in which Snow Leopard automatically detects the mobile as a possible Bluetooth Modem (Mac OS 10.6 rocks!), input the following:
Model: GPRS (GSM/3G)
Account Name: user
We're not done. At the end of this process, open System Preferences and go to the Network preference pane. You should see another option in the list of internet devices called "Bluetooth DUN". When selected, to its right, input:
Telephone Number: *99#
Apparently the mobile takes this as an init-string to connect to the GPRS network. That's it, click on "Connect" and you should be on your way.
The speed is better than dial-up in my opinion, although it's been long since I used the Motorola L6 for that. In any case, it's good to know that I again can be connected to Internet anywhere I need.