Many of the newer 2.4 and 2.6 Linux kernels support Bluetooth natively, but they are not enabled by default. Compiling support for Bluetooth under Linux is a very simple process. If you are used to building a kernel from source using the commandline configuration, or the GUI xconfig configuration, the process is very straighforward.
Under FreeBSD, the Bluetooth stack is developed only for the 5.x-CURRENT branch. You will need to track -CURRENT but you can also use 5.1-RELEASE. Bluetooth support is currently integrated in FreeBSD CVS tree.