[6ren]

http://jsfiddle.net/hYfN3/197/

    elm.onkeydown = function(evt) {
        evt = evt || window.event;
        var keyCode = evt.keyCode || evt.which;
        if (keyCode == '13') {
            var nextid = this.id.charAt(0) + String.fromCharCode(this.id.charCodeAt(1)+1);
            document.getElementById(nextid).focus();
        }
    };

disclaimer: I don't know JS