I use ncmpc, which is a great great curses based client for mpd. However, my multimedia keys won't work with ncmpc. I've been looking around for a while, trying to make them work so I don't have to keep switching to ncmpc when I want to move to the next song etc. I finally found something today. It works!

This is for gnome-shell on a fedora 15 system.

Hit the "hot corner", in the search box write "keyboard" (The search box is really convenient)

Once you're there, create a custom short cut (look at the bottom of the keyboard settings dialog).

Create a new one. Name it "MPC play", and in the command, write "mpc toggle". It will be disabled, click on disabled and press "ctrl + shift + alt +p". Of course, you can put whatever key bindings you like.Similarly, add another for stop, prev, next. Look up "man mpc" for the commands.

Simple, effective, awesome! Cheers!

Edited: mpc toggle added.

Edited: Notifications from mpc

If you want to see what song's playing, you can use a simple script such as this one:

if [ -x /usr/bin/notify-send ]
then
    status=`mpc status | egrep playing`

    # check if playing
    if [ -n "$status" ]
    then
        notify-send -t 5 -i /usr/share/icons/gnome/scalable/actions/media-playback-start-symbolic.svg "MPD: Now Playing -> " "`mpc status | head -1`"
    else
        # is it paused
        if [ -n "`mpc status | egrep paused`" ]
        then
            notify-send -t 5 -i /usr/share/icons/gnome/scalable/actions/media-playback-pause-symbolic.svg "MPD: Paused -> " "`mpc status | head -1`"
        else
            notify-send -t 5 -i /usr/share/icons/gnome/scalable/actions/media-playback-stop-symbolic.svg "MPD: Stopped!"
        fi
    fi
else
    echo "notify-send not installed"
    exit -1
fi

exit 0

Now make a mapping for it, I've used ctrl + shift + alt + w (what's playing now).