Virtual Domains and authentication

[James Satterfield]

I'm having a lot of difficulty wrapping my mind around authentication for a
virtual domain configuration. I would like to use PAM for auth, but I don't see
how to get around the '@' in the usernames. I see nothing in the docs that
address how to setup auth for virtual domain support.
Do any of you have any tips, howtos, advice, config examples?

Thanks,
James.