Greg Burch has made a post on some things to watch out for when using Object.registerClass to set the type of an object to a custom class.
The same thing applies on the Flash Communication Server when using application.registerClass. The constructor for your class must be in this form:
Doing this on the client and server allows you to send custom classes between the client and server and have them automatically serialized and deserialized. This is a very cool feature.