I ordered my 128i on 05/15/10, arrived at the dealer on 07/13/10 (forget the date production started). Ordered my 1M 10/08/11, production started 10/26/11, arrived at dealer 12/01/11. In both cases I received a production number only a few days later but as you can see production starts a few weeks later and you can generally count on about 7 to 8 weeks from order to pick up. The production number is just giving your car a place in the queue.
My girlfriend and sister got their F25 X3s in 5 and 4 weeks respectively but that was when they just started making that new model and those are produced in South Carolina so there is a lot less travel time from factory to the dealer where we live.