As you might know, a pristine N900 supports tethering over USB only. To make it work over BT, you need to follow the instructions for adding a DUN server on Maemo.org’s wiki. Being always in a hurry, I never actually tried it until I came across Philip’s post on controlling Bluetooth DUN with upstart on the n900.
The good news? Using Upstart is the correct approach and obviously works perfectly as far as the data connection is concerned.
The bad news? Apparently, there’s some weird interaction going on with tethering and the phone data counter: the traffic caused by the attached computer is not added to Settings->Phone->Data counter, which is simply overwritten. According to the dataplan monitor applet, before tethering I had used 24MB out of my monthly quota of 3GB but when the connection ended the counter was reading 2MB instead of 26MB. The reset date for the counter was unchanged, though, so I’m inclined to think this is not the intended result.
Bottom line: if you rely on the applet to monitor your quota usage and wish to use tethering, be aware that you need an alternate way to check your situation, such as 3 Wired Portal
EDIT: apparently the data counter is clobbered also when using DUN over USB