We have known for days the AMD driver would be merged into FreeBSD 10 and this week we received official confirmation. As of this SVN revision on Sunday, the latest FreeBSD development kernel now supports this AMD DRM/KMS driver. This work also comes just a short time after OpenBSD received AMD KMS support and that Intel KMS support landed in FreeBSD 9.1.
The AMD driver present in FreeBSD trunk is ported from the Linux 3.8 kernel's Radeon DRM code while the TTM memory management code is slightly newer from the Linux 3.9~3.10 kernel. This AMD KMS driver isn't found in the soon-to-be-released FreeBSD 9.2 but it could still be backported to the FreeBSD 9 series.
Most AMD GPU functionality should work under FreeBSD but additional suspend/resume testing is needed, debugging various issues, etc. The latest Mesa and xf86-video-ati code is also needed for user-space. More details can be found on the AMD FreeBSD Wiki.