The thing that does not make sense to me about the rumor that a 5D MKIII high resolution camera replacing the 1Ds MK IV (or whatever) is the weather sealing. Many pro nature & wildlife photographers comment about the excellence of the weather sealing of the 1D series of bodies. I even recall one comment about a photographer accidentally dunking his camera in the ocean, shaking it off (presumably drying the front element) and continuing shooting. Unless the 5D MK III has that level of weather sealing, I can not see that group of users being entirely pleased with it.
As far as MP count and resolution go, Jay Goodrich, posting on behalf of Art Wolfe, (April 13, 2011 entry http://blog.artwolfe.com/2009/01/equipment/
) says that they print up to 30x40 inches using Photoshop (to size the image for printing) for images captured with Art's 1Ds MK III 21 MP camera. He also notes that Art used a tripod, cable release and sets the camera for mirror lock-up whenever possible.
I am certain that Canon will continue to increase the resolution of sensors, but it seems that, beyond about 24 MP, other considerations may attain greater influence. We shall see.