It's fairly easy to create an icon-only tab (once I figured out some of the nuances). Just supply the icon class along with a non-breaking space for the title. (To clean up the width of the toolbar a bit, reducing the right padding to 3px makes it perfect).

First, that would be a nice feature (built-in support for a properly formatted icon-only tab).

Second, in IE, a tab with a tooltip doesn't show the tooltip unless there is actual text in the tab title.

Finally, on a related note, setTitle allows for changing the tab title and icon, but there is no built-in way to set, change, or remove the tooltip (other than the initial config).