Detect focus initiated by tab key?

I know you have accepted an answer but you could test the button pressed using the following:

$('#detect').on('focus', function(e){
    $(window).keyup(function (e) {
        var code = (e.keyCode ? e.keyCode : e.which);
        if (code == 9) {
           alert('I was tabbed!');
        }
    });
});

http://jsfiddle.net/LPGLm/1/

Edit: change the listener around:

$(window).keyup(function (e) {
    var code = (e.keyCode ? e.keyCode : e.which);
    if (code == 9 && $('#detect:focus').length) {
        alert('I was tabbed!');
    }
});

http://jsfiddle.net/LPGLm/7/