By Hans Muller:
Flex 4 beta provides an interface called IViewport that all scrollable components must implement, and Scroller, a component that uses scrollbars to enable interactive scrolling of a viewport (a component that implements IViewport). This support for interactive scrolling is an essential feature of all graphical user interface (GUI) toolkits. The Flex 4 beta approach is intended to be more efficient than the one used in Flex 3; see the section Comparison with MX scrolling for more details.
Many Flex components, including List and TextArea, include a Scroller and a viewport as part of their skin, so that developers don’t have to worry about enabling scrolling. This article explains how the IViewport interface works and includes a simple Scroller example. If you’re interested in creating your own interactive scrolling components or just want a better understanding of how Flex 4 scrolling works, read on.
Read more at: http://www.adobe.com/devnet/flex/articles/flex4_viewport_scrolling.html