A lot of people here don't think that R&D for new cameras cost money....
If they were to come out with a budget FF camera, it would be a lot like what they did with the XXXXD series... recycle old sensors and add/ cut out some new features.
If there is an entry level FF camera, then the most cost effective way to produce it would be to use the 5DII sensor, and some AF system they already have... ie the current 5D one or 7D one....
They can then sell this at a lower market price without have to spend money on R&D while protecting the sales of a newer tech 5DIII.
It makes much more sense from a market standpoint to use the same sensor (7D, 60D, 600D) and vary the features because that reduces costs.
Along this line, I expect that the new 5DIII will have the same sensor as a 1DsIV. Either the 1DsIV will drop to like 6999 price point or the 5DIII will rise to 3499.