It seems the use of the terms 5D X and 7D X are becoming popular, and I've even used 5D X a couple times myself. I've been digging through all the available material on Canon's forthcoming 1D X, and from what I can tell, the use of the moniker "X" is because Canon considers that particular body to be the "tenth generation" of their flagship camera. I was confused how they could call it the tenth generation, when at best you can count eight separate cameras in the 1D line (including the 1D II N). Generationally, there are only four generations of the EOS 1D line. It seems Canon is actually counting the EOS-1 Series film cameras as well, which stopped at the 1V (fifth), followed by the four generations of 1D digital cameras. That would mean a total of nine EOS-1 cameras, both film and digital, have been released by Canon, making the 1D X quite literally deserving of the Roman Numeral for ten.
Given that, I think its highly unlikely we'll see some of the more wild rumors, such as a 5D X which would be the merger of the 5D and 7D lines, materialize. The "X" designation is not indicative of "merger", its indicative of "ten", and the 5D and 7D lines are still relatively new in the grand scheme of things. There have been the EOS 5 (more commonly known as the EOS A2 and A2e), as well as the EOS Elan, which included the EOS Elan 7, but I don't believe there is any correlation between those cameras and the 5D and 7D as there was with the EOS-1 and the 1D line.
Unless Canon decides to skip a few generations for both cameras (and nine whole generations for the 7D), it seems most logical that we'll see the 5D III rather than a 5D X, and a 7D II rather than a 7D X. I also find it unlikely that we'll see any kind of merger of the 5D and 7D lines...both have their niches, and even if the 5D gets a better AF system, its still unlikely to *replace* the 7D for anything that needs a high frame rate and a cropped sensor for the added reach.