I would assume official preorders for both Revolution and PS3 will start right after E3, presuming they announce pricing and exact dates then.
Just like the 360 did, if I remember right.
Preorder on day one, whatever you do. Who knows if Sony/Nintendo will do a special deal with the Best Buys and Circuit Cities of the world this time around, thus screwing with the more 'standard' (EB and Gamestop) stores when it comes to preorders.