I had the same problem others seem to of have. Disabled RadComboBoxes were ugly when used on a page that had a FormDecorator and a parent Skin. Please see the resolution below:
div.RadComboBox_Vista .rcbDisabled INPUT.rcbInput
background-color: White !important;
color: Gray !important;
Assuming a Skin of 'Vista', this will style the disabled drop down. I made it simple, used the same background color as the table the control was in and choose a Gray fore color.
Hope it helps save someone the few hours it took me to amalgamate a solution.