New BMW Bluetooth SAP Universal Snap In Adapter
 

Apparently BMW has very recently introduced a new Bluetooth SAP (SIM-Access-Profile) Universal Snap-In Adapter. P/N 84-21-2-158-363
It is for vehicles with SA 609 (On Board Navigation System) and SA 633 (Bluetooth Mobile Phone Preparation Business) / SA 644 (Bluetooth Mobile Phone Preparation) starting from 09/2008.

It's not on realoem or getbmwparts yet. However you can get it from Roveractive (item # 170374175056):

BMW Genuine Bluetooth SAP Universal Snap In Adapter : eBay Motors (item 170374175056 end time Mar-18-10 11:37:34 PDT)

According to BMW it is compatible with:
1 Series (E81, E82, E87, E88)
3 Series (E90, E91, E92, E93)
5 Series (E60, E61)
5 Series GT (F07)
6 Series (E63, E64)
7 Series (F01, F02)
X1 (E84)
X5 (E70)
X6 (E71Z4 (E89)

And it is NOT compatible with:
3 Series (E46)
7 Series (E65, E66)
X3 (E83)
X5 (E53)
Z4 (E85, E86)

what does this thing do? what is it for?

SAP is remote SIM access. The car kit takes control of the SIM and uses the GSM radio in the car to make calls, access the phonebook and possibly the SMS also [depending on how the phone handles SMS, storing them on the SIM or the phone memory].

Very few phones support it these days, and the car kits are not Class 1 (5W) mobile devices [for GSM], so the improvement you get in reception is restriced to the external dipole (sharkfin) on an ample Ground (the roof). Also the battery is drained less in the phone during a call.

From the humongous price of the unit, I assume it contains an additional bluetooth radio, that then mimics a phone when talking to the TCU.

It also appears to have an additional SIM card slot for full support of 1992 dual SIM subscribers.