Well, to answer your question, I've been using this "redirect" with parking for a while and a while back I got an email saying to remove one of the sites because of licensing. I explained to them what it was doing and they were fine with it. It's one database and the content is not duplicated on the server.
http://www.leetforum.com/forumz/
http://www.theforumz.com/forumz/
http://www.gameforumz.com/forumz/
http://www.techforumz.com/forumz/
http://www.3dnewz.com/forumz/
The only real problem is that my site use to go by TheForumz so everybodies link is that, but then I changed the name to LeetForum.com but didn't move the site or anything, just parked the LeetForum.com domain ontop of TheForumz.com, but the problem arises when people still use their TheForumz.com bookmark. I assume this could be a problem for all my Google indexes too. Guests that try to do stuff would likely hit a snag somewhere along the way.