The html renderkit seems to ignore the itemDescription on selectItems. More specifically the description is correctly fetched but nothing is then done with it.
This looks to be a one line change in MenuRenderer.java, specifically adding the line:
writer.writeAttribute("title", curItem.getDescription(), "description");
into renderOption(...). This successfully adds a title attribute to the option element and causes the description to be displayed when the select item it hovered over (tested on IE and firefox).
The full text of the changed routine is attached.