While not owning the D800, from my reading of the specs, one of the biggest advantages to DX mode is the frame rate of 6fps vs 4fps, with the grip of course. That's a real advantage for sports and wildlife action. I don't know how quickly you can change from FX to DX mode and back, so that would be a concern. But if you really want the best of both worlds, money no object, then a D800/D7000 or a D800/D3s or D800/D4 is the way to go.
OldPhotos "If everyone possesses some measure of this intangible quality called creativity, photography is unprecedented as an outlet for its expression." - Ansel Adams