One of these tools allows you to prefix a number with a zero. It's in com.adobe.utils.NumberFormatter. The method is addLeadingZero(). It's kind of confusing having two NumberFormatter classes, but I didn't write it so there you go.
There's a lot of good stuff in the project, so if you're looking for something Flex doesn't do it might be in there.
I also wrote my own string padding function that can be used in Flex, AS2 or 3. Please see this post for more details.