I wouldn't be surprised if the 5DIII is just a incremental bump in AF, weather sealing and a "downgrade" to the 18MP sensor to save costs rather than developing a whole new sensor.
The only fly in the ointment with this theory is the purported 36MP in the Nikon D800. Really though is video that big a market that they need dedicated video cameras and bang on about video in the stills cameras? As a stills-only landscape shooter I'm not feeling the love from Canon - seriously thinking of going the Nikon way with a D800 if they don't look after me.
What is hilarious is that Nikon was the low MP one (D700 only 12MP!!! D300s only 12MP!!!). Now Canon wants to jump on the bandwagon Nikon is leaving!?!?!
Technology advances. And will continue to advance. And hopefully, the pixel count will decide to advance in the next offings instead of fail backwards. A 5d mkII is interesting to me. An 18 MP 5D would be boring and useless to me. And a less than 18 MP 7D anathema!
So, anyway, it will be interesting to see where this will go... But, seriously, I can totally see a whole lot of people jumping to the darkside if the 5D3 has only 18MP...