I don't think you can directly compare high ISO noise from the a900 to that of the D3 or D700. With twice the pixel density, I wouldn't expect the a900 to compete directly with the D3/D700 in high ISO.

While Nikon did a stellar job with high ISO noise in the D300, Sony is finally catching on and their new v4.0 of fw for the a700 looks to be very D300 like.