cloneworks:
If you want everything on a single page, perhaps you should do some research on AJAX.
Ajax will effectively allow you to do things like check for username availability, etc so that on the same page you can query different items.
The only part that has me confused with your request is, why when they log in from the single page would you want to have the registration stuff there?
If I don't get what you are asking for, please clarify.