This is my kind of serial (pardon the pun). Ralf Ackermann has been hacking Bluetooth modules into the iPhone and iPod touch, and blogging about it in installments. This mod is different from the built-in Bluetooth in that it opens up possibilities way beyond just connecting a headset:
A Bluetooth stack in user space already existed for quite a while. The researchers at ETH Zurich provide software for their BTnodes. A BTnode has (among other features) an ATmega 128 micro controler and a BT module with an HCI interface that is connected via a UART.
Fortunately, you can do development and real-world tests with just the same software compiled for running on a host system (e.g. PC). The emulator runs in user mode and uses a BT module attached to the serial port.