Great question! Also great to see FW being used in the mix!
Have you read Adobe Flash Platform from Start to Finish: Working Collaboratively Using Adobe Creative Suite 5
There is some advice in there on handling it. With the limitations in FC 1.0, smaller components are the right method to solve this. In FC 1.5 has some round tripping, but it may not be enough.
With FC (CS5) and FB 4 there really isn't a simple (or easy) round-trip workflow, yet. The next versions (Burrito and Panini) are supporting a better process, but here is what we did for creating Adobe WorkflowLab.
The initial design was laid out in FC and then brought over to FB. As we continued development in FB and the designs changed, we would use FC as more of a converter then a design/dev tool. We would bring parts of the design that changed from Illustrator (our designer used this for all assets) into FC. We then converted the design as needed then we either opened Code view in FC and cut and pasted the generated content into FB; or we exported the library and then brought the library into FB. Once the library was in FB we then copy and paste the content into our FB project as needed.
As you can tell this is hardly an ideal workflow, but it was still significantly faster then older ways of doing design integration into Flex applications. One note about this process, we spent a lot of time upfront working on wireframes and creating a designer/developer "contract" to help the parallel development process from falling apart. If you are interested about this process then I would check out our series on PeachPit about how we planned, developed and deployed WorkflowLab using CS5: http://www.peachpit.com/articles/article.aspx?p=1624607
Hope this helps a bit!
@Chris and @James. Thanks for the responses. Seems like you both recommend this book. After reading the ToC it looks like a great book, but doesn't seem to directly address the day-to-day workflow issues at our low levels, but more of a project plan for a Flash App.
I just purchased FW,Catalyst and FB. Anyone have any idea when panini and burrito are set to launch? I'd hate to be a version behind when I just dropped the money on current release.