According to some rumors kicking around on some Far Eastern hardware sites, a dual-GPU version of the GTX 260 is due to arrive from NVIDIA soon into the new year.

The GTX 260 GX2 is rumored to be making its first appearance at the Consumer Electronics Show, on January 9th, with retail appearances following soon thereafter.

The card has not been officially announced, but doesn't seem unreasonable.

It would also make sense that NVIDIA would want to make a GTX 260 GX2, instead of GTX 280 GX2. Due to GPU yields, and the challenges of keeping a hypothetical GTX 280 GX2 cool, the GTX 260 GX2 seems like a better bet. Especially if NVIDIA decides to use the 216 shader core version of the GTX 260, which has been shown to challenge the supremacy of the HD 4870, the GTX 260 GX2 might be fast enough to make it worthwhile. You have to assume that logically NVIDIA would not release a GTX 260 GX2 unless it had significant odds of competing in cost effectiveness to the HD 4870 X2 cards, which have already been out for a while. 

The GTX 260 GX2 will reportedly use a die-shrunk, 55nm version of the current 65nm GT200.