Put this in your .zshrc/.bashrc
ssh $* -t 'tmux a || tmux || /bin/bash'
$ ssht user@host
will attach to existing tmux session, create new if one does not exist or just start bash if there is no tmux available.
Love this tip, a small adjustment I made was to use the users default shell (where it isn't /bin/bash):
getent passwd $LOGNAME | cut -d: -f7
@teamon and then you're just one step away from what I consider to be the ideal ssh+tmux setup: http://goo.gl/XumZS9