(Aside: Your username can be identical to your email, and we'll provide a parameter to that effect, that will cause the default upgrade login/registration process to be equivalent to that of earlier versions on upgrade.)
Now, the question is what we should do with screen_name.
1) Keep it, so you now have both username and screen_name?
2) Keep it, but default it to username - this could be odd if your username is already defaulted to your email, but it could be useful if you let people use something a' la AOL's screen-name for username.
3) Ditch it in favor of username? This has some implications, e.g. are we going to let people change their usernames if they're not authenticated by OpenACS? Also, username is bound to the authenticating authority, so the same OpenACS system may have muliple users with the same username.
I think I prefer option 2.
Other ideas? Thoughts?