Tim,
If you look in your DNN Environment at \DesktopModules\Admin\Security, you will find two files: Register.ascx and Register.ascx.cs. This code is the default registration view that users see. This usercontrol and codebehind is dynamically compiled so if you make changes (like raising a custom event), it will get recompiled by asp.net automatically.
That being said, I wouldn't recommend customizing these files because DNN upgrades will become more complex. But you can develop a new module and use this code as a starting point. Then you can place your custom registration module onto a new page and set that new page to be the Registration page in the Site Settings.
Good luck!
Scott