Lifted from the Canon Rumours site.
Looks like the new 5D MkIII also has a headphone socket and a push button on the mode dial (like the 60D), so it won't turn accidentaly.
5d3_1dx-431x575.jpg
5dmarkiii2.jpg
5dmarkiii-431x575.jpg
The official announcement can't be too far away.
Canon Rumours say it will be announded on the 2nd March!