Perhaps this is already obvious that the 5D mark III is imminent, but out of curiosity, I tried entering "Canon EOS 5D mark iii" in the search box for B&H. In response I get the following message:
We're sorry! B&H does not have any information posted for "Canon EOS 5D Mark III" at this time. See below results for CANON 5D MARK II"
This brings up a couple of points:
1. This error message appears specially crafted for the mark III. The capitalization has changed.
2. If I do the same search for a mark iv, I don't get any such message, just the normal search response that hits on some other content that apparently has 'iv' in some field.
Trying this for the 5DX does not turn up anything.