Someone may have already mentioned this, but how about an option to allow the admin to control where guests can post on the board, so they could have a dedicated "guest posting" area without making the entire board accessible to guests?
Also, something that might be useful (for more lazy members): If a guest posts, have a place where they can enter their username and password before posting, so they can post without having to login? I think Snitz Forums (ASP-based forum) has something like this. I know this sounds stupid, but this would allow members to post using their membername even if they didn't log in to the board, which might be useful if they're on a shared computer...
