Is selectedItem null after the assignment? Maybe your value is not in the dataprovider, but only in a field in the data objects? If so, then you have to search the objects to find the match.
The listitem is getting highlighted but the list is not scrolling to the listitem. If the list is long and the item is off the screen I have to manually scroll to the item. It should just automatically scroll the list so that the selected item is displayed.
There is a separate API to call to do that. The API is different for mx:List vs s:List
It's the s:List.
The only thing that came close in the API documetation was ensureIndexIsVisible. I tried that and still no luck.
l = myLists.selectedIndex;