tmux on mountain lion
That message comes from my reattach-to-user-namespace wrapper program that let’s you (e.g.) access the pasteboard inside tmux. You are using an old version of the wrapper (from prior to Mountain Lion).
The old wrapper does work correctly under 10.8; the message just means that you are using a version from before 10.8 was available/verified. So there is nothing to worry about, but it may get annoying seeing the message every time you start a new tmux pane/window.
To get rid of the message, you should upgrade to a newer version of the wrapper. If you previously installed via MacPorts of Homebrew, just upgrade via that package manager. Otherwise, download the latest tagged source (or pull into your previous Git clone), rebuild it (e.g. type make), and copy it to the location that your tmux default-command setting uses.
Both MacPorts and Homebrew currently package the 1.1 version, which suppresses the message when running on Mountain Lion. The 2.0 version also includes the program’s name in such messages so that the source of any future messages will be more obvious.