Can't personally see why you would want tooltips on select options but, if you must, the only way would be with CSS. Put a div inside each option (this might fail validation, mind you), and use some CSS like this:
option { position: relative; }
option div { display: none; position: absolute; left: 100px; width: 150px; height: 35px; }
option:hover div { display: block; }