Emacs Ctrl-x ctrl-s command not working in Terminal.app
I just ran into what I think is the same problem when trying to use Ctrl-S under Vim in Terminal.app. I found a related tip indicating that by default, Terminal.app reserves Ctrl-S for old-fashioned XON/XOFF flow control.  Adding this line to my .bash_profile -- or just entering it at the prompt -- freed up Ctrl-S and Ctrl-Q for use with Vim:

stty -ixon -ixoff