Hi Greg,
I have been following this thread and reading the replies. It appears to me that the general opinion is "no junk mail" on the forum.
I have a small (300 or so person) group that I mediate on MySpace. The way that I have it set up is that there is a "forum" section and a "bulletin" section.
I allow businesses and organizations to post their profiles within the group. The rules for posting are that discussion topics are posted on the forum and that announcements, advertisements, etc... are posted on the bulletin section.
This eliminates the Junk Mail from the forum section and presents advertisements and announcements on a more appropriate format in the bulletin section.
This arrangement appears to be working pretty well.