I've read through this thread, but no one has really talked about the 6FL retrofit kit from BMW. I heard months ago about this, but that it was only for "business" audio systems and never really became available for the "other cars." I got a copy of the BMW installation instructions, which show all the parts, the wiring diagram, and their locations.
However, these instructions specifically talk about professional navigation (SA609) and the voice input system (SA620.) This looks like it would work "for the rest of us." This kit is designed for the 1 (E81,E82,E87,E88) and 3 series (E90,E91,E92,E93) cars.
For this kit, it talks about two retrofit kit part numbers: 65 41 0 443 678 and 65 41 0 445 465. I'm not sure which one is which and if they are available as was talked about earlier. (Tischer doesn't show anything on these p/n's, neither does REALOEM.)
If I had a way to post this .pdf, I would so you all could read it. However, if you PM me, I'll email it to you..its approx. 2 MB.
For whoever asked, the USB hub is located (in this retrofit) in the driver's side footwell inlay.