I just want to say that my GA-Z68X-UD3H is working in 10.6.8 flawlessly, as far as I can tell.
I started fresh with a 10.6.0 install using onboard graphics. After a successful 10.6.7 update, I plugged in my Radeon 6870 and setup ATI drivers. Everything worked, so I ran 10.6.8 update and once again everything went just fine. Afterwards, I did need to run Multibeast to enable realtek ethernet and audio rollback.
I've tested the system with Final Cut Pro X, Aperture and iTunes 10.3. Also, a cheap, generic bluetooth dongle has enabled my Apple Wireless Keyboard and Magic Mouse.
btw - I initially tried applying the 10.6.8 combo update to my 10.6.0 install, and got a KP during the update. Also, I ran into install problems early when I was using the wrong SATA ports for my HDD. I switched to the bottom white port, and this fixed the issue.
Thanks to OP for the step by step guide!