At the price level it will be at, it MUST have a noticeably better AF than the Nikon D700 and slightly better high ISO performance. To be honest, all of the upper end DSLRs now have very very good IQ and the differences will be small. What will make the difference is the glass. There are newer lenses that are coming out and it would be better to spend the money on that for my 5D, rather than the 5D replacement.
If the 5D successor is over $3,000 USD, then I would have to consider the 1DMkIII or wait until the MkIV.
