Ecoli,
If it's not too late, I recommend using DadaMail instead of mailman. It is waaaay easier to use, more professional looking to both administrators and users. I use both systems, so I can speak from experience. It has easy signup, double opt-in, and a nice clean interface.
The default DadaMail setup is announce-only, which sounds like what you want.
My two cents. Good luck!
-- Dave