The xf86-video-ati 6.12.5 is the new stable update that carries bug-fixes and other work. Some of these changes include addressing a potential memory leak, fixing monitor issues, and updated quirks / IDs. The xf86-video-ati 6.12.5 release announcement with its change-log can be found on the mailing list.
More important than xf86-video-ati 6.12.5, however, is the new xf86-video-ati 6.13 pre-release (v6.12.91). Found in this pre-release and the soon-to-be-released xf86-video-ati 6.13.0 X.Org driver is kernel mode-setting support for all ATI Radeon graphics processors (going back to the ATI R100) when used in conjunction with the Linux 2.6.33 kernel or newer, DRI2 support for all GPU families, initial ATI Evergreen / Radeon HD 5000 series kernel mode-setting support when used with the Linux 2.6.34 kernel, user-space mode-setting support for the R600/700/800 GPUs, R600/700 2D EXA and X-Video acceleration support, UMS power management controls, textured X-Video controls, DisplayPort support, improved Zaphod mode support, and a CustomEDID option for injecting a monitor's EDID information from a file rather than probing the monitor itself.
The xf86-video-ati 6.12.91 release notes and change-log can be found on the X.Org mailing list. It's been quite a while since a new major release has took place for the xf86-video-ati DDX, but version 6.13.0 should change that soon.