When the ATI Radeon HD 4890 launched earlier this month, everyone knew that it would only be a matter of time before the 1GHz core clock speed was achieved in the overclocking community and it did come rather quickly. After all, a 150MHz increase isn't much these days given the vast array of air and water cooling products on the market.
Now overclockers are getting a little more in depth with the 4890 and starting to push it a little bit further with some of the more extreme forms of cooling, liquid nitrogen and dry ice. So far liquid nitrogen has enabled the card to be pushed to 1.13GHz with a modest voltage increase.
Dry ice has proven to be a little more effective, raising stable clock speeds to 1.2GHz with a voltage of 1.425V. ATI's Catalyst Control Center won't allow the GPU to go faster than 1GHz even, but VR-Zone has a very useful guide on just how to get the voltage a little higher in case you are thinking about giving it a shot.