I recently ordered a couple of serial Bluetooth module on Ebay.
You can find the same board with different names and different flash memories on the net but they all are based on the same design. The main chip is a CSR BC417143 a.k.a. BlueCore4-External connected to a 8Mbit parallel Flash memory containing the firmware.
Here is the system architecture of the chip from the CSR datasheet:
The BlueCore4-Ext is a generic Bluetooth 2.0 EDR chip with a bunch of interfaces. The USB interface allows to use it in a USB BT dongle design. When integrating it in a microcontroller-based design, you could use the serial interface instead. The SPI interface allows the programming and configuration of the chip using CSR tools.
The provided firmware implements a Serial Port Profile based on a RFCOMM stack. But in theory this hardware could run other firmwares from CSR, for example a HCI stack to connect the chip as a generic BT USB dongle.
The module hardware is very small. The footprint is made to be soldered on another PCB. A serial breakout board is available with RX/TX connections and a 5V to 3.3V regulator. As I would like to tinker with the module to investigate its capabilities, I instead designed a breakout board allowing access to all the pins on a standard header.
I just ordered a batch of breakout boards from Olimex. I should receive them in a few weeks. If anyone is interested to get one,
leave me a message. [Update 2011-05-06] Ordering informations are on the Shop page.
Here is the module datasheet provided by hongkong_electronics
BC4_Bluetooth Module Datasheet.zip