There's something about a 5D MkIII before the 1Ds that just doesn't sit right. The current 1Ds is probably not as good as the 5D MkII in terms of absolute image quality, if a 5D MKIII comes out then many photographers will buy it, because then it will far eclipse the 1Ds MkIII and be Canons top camera. Inevitably this will take away from sales of the flagship model when it is released. If the release was the other way around Photographers would have a choice of a brand new flagship model which outperforms every other camera in the range, and for those who can afford it, there's no other real option but to buy it. A 5D MkIII give the chance for those unable to afford the 1Ds price to buy into something close (probably the same sensor I can't see Canon developing two different FF ones).
Having said that Canons marketing department have made some spectacular howlers in the past!
I terms of sales of 5D MKII and D700, both models are a little long in the tooth, but in todays economic climate it's hardly surprising that people are not spending big bucks on what many would see as a luxury.