Upgrade to 40D focussing? I think that's a downgrade! hopefully it'll be an upgrade to 7D focussing. As for the body the shutter assembly will not fit in a smaller body, so it's about as small as it's going to get. This is a pro grade body so I don't foresee a move away from the current alloy shell type.
Can the centre spot keep up with the 70 - 200mm f/2.8? I'm not sure I understand this, I think it's putting the cart before the horse, the camera drives the lens at the speed it can (the 1D series drive them faster) but there autofocus components in the lens as well as the camera body.
One night a friend with a D700 and I with a 5D MkII both with 70 -200mm f/2.8 lenses on tried out the autofocus, the D700 stopped quite a way before the 5D MkII did. Although this is only a test of the light sensitivity of the autofocus it does show that the Nikon is not all that it's cracked up to be.
There's about one stop of usable sensitivity between the 5D MkII and the D700, I wouldn't expect that to be enough to justify a change.
The 5D MkII is not a sports / action camera, the 1D MkIV would be a fantastic choice and with its 16MP will give a decent resolution.
I think Nikon dropped the ball with the D3 & the D700, it's 12MP sensor is not really comparable with Canons, and the Expeed image processor is not in the same ball park, the only reason they have remained a viable player in the market has been the support of older reviewers who have been Nikon fans since they were top of the roost in the days of film, and have a soft spot for them.
Friend wants to sell the D700 and move to Canon so if you're interested ...................