Ok I feel stupid... I've got the logic wrong. I should have had the if statement inside the focus function.
$(document).ready( function() {
var text = '<div style="color:#72008c;font-size:13px;">\n\n</div>';
var el = $('textarea#Form_Body');
el.focus( function() {
if( el.val() == '' ) {
el.val(text);
}
});
});
This works perfectly now.