At Design Time, adding a DropDownList inside a containing component and then trying to add items to that DropDownList via the ListItems Collection Editor is problematic: Clicking the "OK" button inside the editor closes the editor, but does not update the aspx source file--no items are added to the dropdownlist. It would appear that some update event isn't firing.
If the DropDownList is not inside of a container, adding items via the list editor works correctly as expected.
I have verified this (what appears to be a) bug on two systems, one with XP SP2 and one with Vista 32 bit. Both VS installations have the recently released VS2008 SP1 installed. I at first thought it was a third party component problem, as I encountered it when placing the DropDown list in an Infragistics UltraWebTab container. But then I also experienced it inside a Telerik RadMultiPage RadPageView component, which led me to try it inside of standard MS container components that come with VS2008 and I found that the problem existed there, too.
So, I now believe it is a problem with the DropDownList component itself or some base component it is derived from or how the ListItem Collection Editor interacts with it.