Well, I think he's right. You cannot detect if a peripheral has been attached to the N900; you need to be in a "special" mode which forces the PHY to be powered, adequate pull up/downs, ....

It is theoretically feasible to be _always_ on this especial state, but I wonder about battery life...

Of course since you plan to use a self powered "docking station" you could use vbus sense for the task.