Nordic Hardware published their overclocking review of the Radeon HD 2900XT yesterday and unfortunately they didn't have the time to include any tests with liquid nitrogen. They've tried now, but only come to the conclusion that it's simply not feasible to try and overclock the R600 with liquid nitrogen and the current LN2 containers available. It's too damn hot!
However they did manage to reach a new record high 1230MHz GPU frequency and a whopping 777GFlops performance (a quad-core processor is <50 GFlops).
"That's exactly what Kinc did with the Radeon HD 2900XT and with the extra cooling the nitrogen provided a new top notch result was noted in core frequency, namely 1230MHz stable enough to run a round of 3DMark 03 which makes it the highest clocked graphics processor in the world so far."