AMD will not officially release a Radeon HD 4890 X2 graphics card because it would use too much power, but according to Bright Side of News some ATI partners are prepping a dual-GPU version of the RV790.
However, our sources in Taiwan claim that the ultra-overclocked board e.g. SuperRV790 isn't exactly what they are interested in. AMD GPG [Graphics Products Group, e.g. division F.K.A. ATI] recognized the need for even more powerful part, and gave the thumbs up to partners that want to do the 4890X2. In a YouTube interview for PC Games Hardware, Sasa Marinkovic [Product Manager for Consumer platforms: CPU, GPU, Chipsets] practically confirmed the X2 part. You can fast forward to six minutes into the interview for the "we're going flat out to capture the crown" statement.
Current king of the hill is GeForce GTX 295, dual-GT200b based card. Our sources claimed that they are ready with their own 4890X2 designs - power issues, in worst case can be addressed with two 8-pin connectors. If this truly comes to life, a 4890X2 would be the most power-hungry card on the market, with a potential to eat 375W [75W from PCIe slot, 2x8-pin for 300W].