Frankly, I don't even know the difference. Maybe what I've been trying so far isn't "tethering" at all. I guess I wouldn't care about the details as long as it works and is half-way convenient.
I don't have my N900 at hand, so I can't check right now: Does this mean, that by default whatever kind of "network" won't work on the N900 via bluetooth? If so, that might explain my problems. But then I don't understand why it worked years ago.