A mysterious ATI driver developer known only to the world as 'Chuck' has hax0red teh drivar megahurtz to allow Radeon X1K boards to render Oblivion in 'HDR' rendering mode with multi-sample anti-aliasing on. A combination of features only found on the Xbox 360 version so far, 'HDR' with multi-sample AA is orthogonal on the latest Radeon hardware, so Chuck's madskillz made it happen.
Read about it here and comment in this very thread.