Strange that regular DUN gives you issues @sulu. It worked for me just fine max 2 years ago. Unfortunately I'm reduced to a Thinkpad X40 that has dead bluetooth chip and dead USB ports, so I can't re-test

Perhaps you have something unusual going on in your N900 bluetooth config? e.g. bluez from -devel? attempts to use BT keyboards or play with bt audio sinking? (at least i know i've played with all those at some point)