In reference to the "Switching Basemaps" sample in the API for Flex 2.0
Since the buttons for changing basemaps are done using an <s:ButtonBar>, instead of a <mx:ToggleButtonBar> the user can reclick the selected button causing the basemap to disappear. Essentially, no basemap is selected. I tried a variety of fixes to this, but it seems to be more effort than its worth (not to mention I'm not much of a programmer). Maybe there's an easy solution, but I would recommend switching back to the ToggleButtonBar.
I'm definitely curious to see how to fix this and still use the buttonbar.