"too many pixels for a 1D and too few for a 1Ds"
Why "too many" for 1D? The 1D4 has a 16MP crop sensor because even Dual DIGIC4 couldn't process 10fps at 21MP. If dual or single DIGIC5 can process the new FF sensor resolution at 8-10fps then there is no reason to have two 1D bodies. Producing and marketing two bodies is more expensive than one so there has to be a compelling reason for Canon to continue with a 1D and a 1Ds.
Why "too few for a 1DS"? That would be a 50% increase over the current 21MP.
I agree though about the 5D3 taking over for the 1Ds... perhaps 1D5 and 5D3 will have the same sensor and the main difference will be Dual DIGIC5 in the 1D5 (9fps and best in class build quality/ruggedness) and single DIGIC5 in the 5D3 (4fps). 7D2 will also be single DIGIC5 but crop sensor of about 20MP will allow 8fps?
Why should the 5D3 get "only" single DIGIC5 chips? Remember a HUGE part of the success of the 5D2 is due to its video capabilities. I know lot's of people hate video in DSLR, but let's face it, times are changing, and I am sure the Nikon D4 will have improved video capabilities over the D3s AND the 5D2 !!!
And my point is, I know a lot of people are expecting improved video capabilites from the 5D3, maybe RAW movie mode, maybe 4K movie resolution, maybe both. Both putting huge demands on the computing capabilities of the 5D3 = Dual DIGIC 5 !! I am one of these hoping for both. If not, I may be forced to go to Nikon if the D4 delivers as rumored.
Also, with USB 3.0 and UDMA7 memory cards (100MBps + write speed), which many expect on the 5D3, Dual DIGIC5 chips may be required to be able to handle the transfer/write speed, as well as the buffering/cache on the camera. (Also, when working with timelapse, as I am, and many many more are doing every day now, we are limited to 1,5fps withe the bottleneck of 32MBps write speed to UDMA6 cards on both the 1Ds and the 5D2).
Bring me the good stuff Canon!!!