One of the projects I am currently working on of course calls for a user to have the ability to login/register. This is my first time playing with Flex 4 since I have been so heavily building other projects out in 3. I was really happy with the way States now work in 4, I have often heard about it but never really got to toy with it.
Used Flash Builder as well, and was happy to see the code completion revamped, as well as the compiler was a step faster. The only con was the code highlighting, however, since it is Eclipse, it is easy to turn off.
Anwho, take a look at the Login And Register form here.
You can right click on the app for the source view or click here.