The more I think about it, the more I think the 3D is going to happen. People are screaming out for a full-frame 7D equivalent - rugged, compact, weather sealed, with better AF. (OK I am - hopefully others are too!). An outdoor/landscape shooter's dream, for those who can't afford or don't want medium-format digital.
If they made the 5DIII like that it would do two things:
1) push the price right up for that model
2) cannibalize sales from the 1D line
So the market is ripe now for a model that sits in-between the 5D and the 1D lines. There's a big hole in their lineup that they currently aren't filling.
The 5DIII will be the cheapest full-frame offering. The new version will bring more megapixels and better AF from the 7D but not much else, except for perhaps newer video-specific features. It will compete aggressively on price.
The 3D will basically be a full-frame 7D. It won't have integrated vertical grip. It might even borrow some other features from the 1D line but not enough to cannibalize it, but will be a price premium above the 5DmarkIII line, the same way the 7D is at a premium above the 60D. Shutter speeds will definitely be held back as this is a 1D feature - expect a single Digic V for the 3D.
The 1D and 1Ds series will merge together into a fast-shooting, fully-featured, top of the line model - everything we have now but with more Megapixels and the updated interface from dual Digic V. What Canon will do with this camera is enable a toggle to go into 1.3x crop mode, discarding the outer-most pixels which will allow a faster shutter speed, so you'll have a 1D and 1Ds all in one.
Thus we'll end up with a nice matrix of cameras:
Rebel / 60D / 7D for crop sensor
5DmarkIII / 3D / 1DmarkV for full frame
I'll be going for the 3D for sure - maybe upgrade both of my 5DII's to them. Bring it Canon!