Its not behind, it is better, but not by enough margin, Until ATi's release, the GX2 is best bang for your buck.
In bioshock the GTX 280 performs much better than the GX2, in other games the GTX is only slightly better, and in other games the GX2 is better.
But the difference is the price, the GTX 280 costing on average 100 USD more than a GX2 is just too expensive for it's performence, Of course it all depends which game you want to play and at what resolution.