Hook the Keydown Event in the TinyMCE Post Editor

the TinyMCE Editor has its own keydown event handler and its hooked to a function on initiation so to do that you can create a tinymce plugin or use the wordpress initiation of it with tiny_mce_before_init hook like this:

add_filter( 'tiny_mce_before_init', 'wpse24113_tiny_mce_before_init' );
function wpse24113_tiny_mce_before_init( $initArray )
{
    $initArray['setup'] = <<<JS
[function(ed) {
    ed.onKeyDown.add(function(ed, e) {
        //your function goes here
        console.debug('Key down event: ' + e.keyCode);
    });

}][0]
JS;
    return $initArray;
}