Admin,
Heres the reason why this is not a good idea.
The requirement of having to register and choose a username will make it much easier for people to be tracked down, be it they choose a username such as surferguy29 (while it doesn't make any sense at all, you will get some people doing this) and the person happens to be a surfer buff and they're around 29 years old and they talk about something specific. This will just make it way to easy to be caught and I'm sure that you had this in mind when you setup up this message board knowing that people need to keep their anonymity, especially on a board like this.
So for the record, I am against this.
Thank you.