Ati doesn't perform as well as Nvidia for many reason, but I doubt any of them are technical. They may just shy away from competing in the highest part of the graphic card spectrum, because let's face it, it's a fairly small minority of the market. The 3870 stands as it is; 85% the performance of the 8800gt, for a portion less. It's true that the 3870 has yet to fall and meet it's MSRP, which is another piece to account for.
The cheapest 3870 I could find on newegg is $240. 15% extra performance for only 30 bucks extra is a no brainer, I could call that in my sleep. I stand by the 8800gt, the 3870 is great, but the prices need to fall (which they will) just abit more before it becomes a head-on combatant.