To be honest it does makes some sense not to update the iDrive in the X6 right now, I think the X6 is still really fresh and an update such as this so son after the X6's launch would probably upset many existing x6 owners unfortunately the X5 is it's cousin and both go hand in hand in decision making..... On the other hand, it's in BMW's best interests to make the transition of this new feature to the entire model range in a timely manor and i think March 09 sounds very viable and realistic.
Another note, I personally wouldn't be listening to any dealers right now for their opinion on this, i think they need the sales now not in a possible 5 month time...