There are multiple problems with tmux on OS X. pbcopy and pbpaste, for instance, are entirely unusable. Even if tmux has some nice benefits over screen, it probably isn't worth it to the Mac user to switch.
There's a similar-sounding problem in the X11 environment on traditional Unix systems: the current default graphical display is in the $DISPLAY environment variable, while authentication information is in ~/.Xauthority. GNU Screen tends to blindly inherit those settings, so you can freely start GUI apps from inside screen and have everything work... until you try to reattach screen from inside a different GUI session…
I have a wrapper script that reproduces tmux's behavior of automatically updating a set of environment variables. It goes a step further and also updates them in your running bash shells. For this to work you have to register a signal handler in your .bashrc that sources a file with the new environment. The script is available at https://github.com/sciurus/splatbang/blob/master/rescreen