I think that we are talking about different things - API and design. We want to be as close as possible to the MS API, hence we create our API and behavior like theirs. That said, I am afraid that we cannot change the behavior of the IsReadOnly property - it is intended to make the inner TextBox in the combo read-only. We could add another property to change the control as you describe, though.
The background colors and the themes of the Telerik controls are another story. Some Telerik themes (e.g. Vista) are designed to copy an existing standard theme (Aero), but some themes like Office do not have exact standard counterpart and our designers decide how the controls should look. In other words, the different read-only background of RadComboBox in Vista can be considered as a bug in the theme, that we could fix with the upcoming internal build this friday. I personally would prefer Vista and Office to be virtually the same with just some color differences, so I guess we could change the Office backgrounds too. In other words, after the fix, a read-only editable combo with Vista theme would look like a text box with a black triangle on the right side, exactly like the WPF ComboBox with Aero theme. With the Office theme it would look as a text box with a button on the right side. Is this OK?
I appreciate your feedback on our themes. This week we will be reviewing all themes and most probably we will find other similar glitches and bugs, that will be fixed with the upcoming releases.
the Telerik team