Narciso Jaramillo released an informative article on the Flex Developer Center with various ways to customize the new Halo Aeon Theme using Fireworks, Photoshop or Flash. NJ also has a blog and focuses primarily on Flex topics so be sure to keep an eye on it for more tips and tricks with Flex!
I’ve also seen a Fireworks AutoShape prototype that will allow you to export your graphics for use in Flex as 9-slice. Stay tuned for more information on this mysterious AutoShape. 😉