If you want to implement lazy loading (when the user scrolls to the end of the list it triggers to fetch more items) for a spark List control you have to use a workaround right now.
Usually you would add an event listener on the scrollers verticalScrollBar but due to a bug it does not fire events if the user scroll with the mouse wheel or uses the arrow buttons. The bug shows a workaround by listening to the PropertyChangeEvent of the scrollers viewport.
Here a little example app:
The bug has been fixed for Flex 4.5. If there is a better workaround for Flex 4.1 please let me know.