I've no inside knowledge from Canon but based on the Canon 600D that I am currently testing I would fully expect the following features to all be part of the Canon 5D Mark 3 when it is announced.
- Tilt/rotate LCD screen
- Flash controller built in as per the Canon 600D
- Video zoom up to 10x
- Video modes to include 50fps/60fps at full HD
- Additional JPG modes for smaller size files
- Convert RAW to JPG in camera applying various settings
- ISO up to 102400
As the Canon 5D was the first affordable full frame DSLR and the Canon 5D Mark II was the first DSLR to feature full 1080p HD video there is also likely to be an additional feature in the Canon 5D Mark III specification or spec sheet that is completely unexpected.