There's a memory leak in the DropDownList in Flex 4.1 (build 126.96.36.19976). If you set the dataProvider you'll find you can't subsequently clear it from memory and it won't get garbage collected (and nor will the container etc. that houses it).
Adding the functions below provided us with a workaround (in MXML set "preinitialize" to the onPreInit function):
private function onPreInit():void
protected function onRemovedFromStage(event:Event):void
this.dataProvider = null;