Hi Davoc Bradley,
In IE6 there is a problem when using SELECT elements and the workaround which should be used in such cases is to add and IFRAME behind the element which should overlay the SELECT element as you also noted.
The RadToolTip has such an IFRAME in order to workaround this browser bug and the problem should not exist. Please, try setting the RadToolTip's Overlay property to true and test again /make sure you have set explicit size as well/.
the Telerik team