I recently had the privilege of attending a customer-training session given by one of our Flex architects. The session covered component creation in Flex. I volunteered a component I wrote a while back as a test case to have our expert help us write better components.
This gauge component is one outcome of that session (another is an article I wrote which will appear on the Macromedia site early next year). Feel free to download the component. It includes some skin files, too. I placed comments throughout to help guide you when building your own components.