Hi there. I had to write up a detailed how to manual for some folks here at work so I figured I'd just post the link in case anyone wants to borrow it.
You'll probably have to edit it a little but it should be a good starting point. It's in ms word format. Hope it helps someone.
[broken]pub/phplist/import_new_users_pub.doc
[new link] http://docs.phplist.com/ImportUsersWalkthrough
[EDITED BY MODERATOR: Previous link was broken. Replaced by working link]