For whatever reason, the GTX 280 scored abysmally in the Vertex Shader performance test in 3DMark06. This is probably not indicative of actual performance however -- 3DMark06 is getting somewhat old now, and probably did not evaluate the GTX 280 accurately in this particular test.
In the rest of the results, the GTX 280 puts it good results, generally scoring around the 9800GX2 and HD3870X2 levels of performance.