Catalyst 10.6 for Linux offers up RHEL 5.5 production support, official support for OpenGL 3.3 and OpenGL 4.0, and 2D performance enhancements. AMD was quick to support OpenGL 4.0 on their AMD Radeon HD 5000 series "Evergreen" hardware via their proprietary driver, but now with Catalyst 10.6 that support is officially there. The older R600/700 ASICs only support up through OpenGL 3.3. There is not, however, any early OpenCL 1.1 support in this month's driver.
The 2D performance enhancements in the Catalyst 10.6 Linux driver are via officially rolling out their new 2D acceleration architecture. We first talked about this new ATI 2D architecture back in February and that it's actually derived from their Windows' Direct2D code, but four months later the support is now stable and plays better with Compiz and other visual effects.
In my absence there's already a lengthy discussion for Catalyst 10.6 on Linux (along with download links) found in this Phoronix Forums thread.