Good and bad news....

The good news is we're pretty sure what the "problem" is. The bad news is there likely won't fix unless it comes from Apple.

Given the symptoms and details, this looks like a TMDS coherency issue. TMDS Receivers made by Ti and Si are supposed to be compatible but in reality need some slightly different support in the TMDS transmitter. On some panels (using the TMDS receiver the graphics chip is tweaked for) , noise will be seen at higher resolutions (generally at 1600, but can also be at 1280).

With some chips, we can toggle between coherent and incoherent modes for our TMDS transmitter. On our retail products with such chips, customers can use our ATI Displays control panel to change this setting themselves (this is also found in the Windows CP) - it's labeled something like "Alternate DVI Mode" or some such. Unfortunately there is no way to toggle this in the Apple OEM driver. And it would be up to Apple to request this feature and/or implement their own control for it.

There might already be a bug report filed at Apple about it. My suggestion is to call Apple and let them know this is a problem for you.

Bruno