I am using win7 x64 and the problem are the drivers. On my previous setup (2x4890) i just needed to install the driver for my monitor, then set the desktop refresh rate to 100hz and then 100hz appeared in CCC. And i am not alone with this problem. Will call AMD first thing on monday and if they dont fix it in 9.11 (using the beta now) then i see no reason not to switch to a GTX295 even tho i dont want to.
If i insert my 8600gt into the PC with the 5870 in the first slot, and connect the display to the 8600gt everything is OK.
BUt when connected to the ATI card its fooked up.
I am really pi55ed that ATI has not given support for this.

It doesnot even work in XP, mate. A member with the same problem reported it on another forum.