mpc-pause was created to emulate the --pause-play command line feature found in xmms.

#!/bin/sh
# vim: ft=sh
MPC=/usr/bin/mpc
if [ "$($MPC status | grep pause | cut -d ' ' -f1)" == "[paused]" ]; then 
   $MPC play 1>/dev/null
else
   $MPC pause 1>/dev/null
fi

You can bind a key (using your desired method) to the Pause key on the keyboard, for example.

This is now done in mpc (known as mpc toggle)