So if you go to the multiplayer window in 1.6.4, and have a server added that has been updated to 1.7.2, it will show as 1.3. I attached a screenshot of the 1.6.4 screen. Apparently you can't take screenshots at the main screens so I recorded a video. It is unlisted on my channel at http://www.youtube.com/watch?v=cAHbZ_8lZ68. This issue also happened with 1.6 servers with 1.5.1 clients.
Duplicate of MC-34763 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.