Just found some details on the wiring behind the ACC system. http://www.bmw-planet.com/diagrams/r.../e90/index.htm
Search for the term "Active Cruise" and you'll see several repeated sections that show the wiring diagram and overview data behind the ACC system.
I found some cheap sensors (relative to the USA price of $2k+) on Germany's eBay site:
Based on my research, in order to even consider an ACC retrofit, you'll need to ensure that your E90 already has the dynamic cruise control feature (BMW option S544A) which applies the brakes during cruise control to regulate speed.
For me, the biggest hurdle in this retrofit is the additional wiring required. If anyone has info on this, please share.
SPECIAL REQUEST: If you know anyone that already has this feature installed on their E9x, try to get some photos of the wiring harness where it connects to the ACC sensor itself. Also, try to get some photos of where the ACC sensor harness taps into the existing electrical system and/or control module.