For the past few months, the open-source Linux Radeon KMS driver has been ported to Linux. It's shown signs of life but still isn't fully working or in a state where it will be merged to the mainline code-base in the near future.
An update on the current AMD GPU FreeBSD status was shared yesterday and it comes down to working on a BIOS size issue, kernel module loading/unloading is working, the X.Org integration is "DONE", for at least Radeon HD 4000 series GPUs RandR and X-Video is working.
When it comes to more of the Radeon user-space running atop this ported Radeon code into the FreeBSD kernel in combination with the Mesa/Gallium3D driver, glxgears is working but some WebGL demos hang. Using VDPAU on Gallium3D is also failing. Another issue within the mode-setting driver is that HDMI connectors are also currently failing.
More details as to the current state of the Radeon open-source driver on FreeBSD can be found via the FreeBSD Wiki. After Intel KMS was merged for FreeBSD 9.1, hopefully some of the Radeon support will be ready for FreeBSD 10.0, but no time-table has been confirmed yet.