I'm having serious doubts that many people AT ALL will be buying GTX 280s once the HD 4870 and 4870X2 come out. The probability of the 4870, thanks to having gratuitously higher memory bandwidth courtesy of GDDR5, should be quite competitive performance-wise to the GTX 260, very likely beating it; on paper it should beat it, and performance results of the 4850 imply that it does, unless there's something wrong in the RV770 that means that it would not gain much from having a lot more memory bandwidth after ALREADY seeing a tremendous boost in shader and texturing power.
And with the $280US 4870 duking it at least within range of the GTX 260, $500US for a 4870X2 would pose a real monster of an opponent for the GTX 280; there's pretty much close to zero odds that it wouldn't best the 280, and by a margin that would definitely be visible... And hence be large enough to show across the board.
Of course, had I already a G92-based card, I'd probably be fine with it now. But as it stands, my X800XT's starting to really show some rusty spots on it, so upgrades are definitely on my mind.