by looking at "libatiadlxx.so" of the latest Catalyst Linux drivers (11.5) you can find some functions which indicate "powertune" support under Linux (thanks to krumplee for the hint). These functions are not (yet) documented in the official ADL SDK.
There is only one little problem. I only have a HD4870 card, which obviously does not support "powertune" at all. So it would be nice, if someone who owns a more recent HD6xxx card could have a try and report back if it works.
Here is my guess, how the ADL function definitions could look like:
typedef int ( *ADL_OVERDRIVE5_POWERCONTROL_GET ) (int iAdapterIndex, int* iPercentage, int* ???);
typedef int ( *ADL_OVERDRIVE5_POWERCONTROL_SET ) (int iAdapterIndex, int iPercentage);
typedef int ( *ADL_OVERDRIVE5_POWERCONTROL_CAPS ) (int iAdapterIndex, int* lpCaps, int* lpValid);
typedef int ( *ADL_OVERDRIVE5_POWERCONTROLINFO_GET) (int iAdapterIndex, ???);
If anyone has further information, please let me know.