If D800 rumors are true and it will be 36mp, I can't see canon releasing a second 18mpx body as the only option. But I can see it being two different cameras as it has been suggested many times before.
If they announce a 18mpx 5DmkIII, and D800 is announced as 36mpx, canon will lose lot of sales. (not necessariliy jumping ship, but people simply won't buy it).
Only way a 18mpx ff camera will be announced first is with a new name. I can see a new body, say 6D, being announced as a ff 18mpx, high iso, great af FF camera priced at or under $2000. This way, people can still think that a 5D with a higher mpx may be coming as well.
Otherwise, I think next ff camera from canon will be a high mpx camera, call it 5dmkIII or 3D, and it will be priced $3000 minimum.
There is easily enough gap between 7D and 1DX to fit 2 ff cameras. Having a $2k ff camera will be great for marketing, attracting newcomers and stealing customers from likes of sony.
And lets be honest, 18mpx FF camera will be more than enough for your average prosumer... even for landscapes. If you are really dedicated and want the highest mpx you can get, then you wouldn't mind investing a bit more and going for a higher mpx alternative at around $3k...
So to summarise, I think there are enough room for 2 FF cameras - one cheaper more general purpose, and one more specialised and more expensive.
And just to remind again, I am basing all this to the fact that D800 will be high megapixel.