A coworker of mine on the Flash Authoring team, Tareq AlJaber, has an excellent blog at flashauthoring.blogspot.com where he explains how you can take advantage of some of the classes in the Flex.swc and use them in your Flash applications.
For more information, see the following posts:
As a long time Flex, Flash, and ColdFusion user I was very intruiged after reading Tareq’s blog, and within minutes was using the Flex ObjectUtil class in Flash CS4 to trace out large objects and arrays, using the [Embed] metadata to embed PNGs and assets from library SWFs into Flash CS4 components, using the Flex HTTPService classes to load remote XML and text and display it in the Flash TextArea control, and creating custom label functions in Flash which used the Flex CurrencyFormatter class. The best part is that it was all very easy! Just point to the correct Flex SWCs, import the correct mx.* packages, and then start coding.