I used to work with a guy on a few tag-board websites, so I have had a lot of experience with them. Just a couple of things I would comment on. For starters, you have got a lot of competition in this area (tag-board.com, tag-world.com, tagbox.net, cbox, ect ect) theres a lot of big Tagboards/shoutbox websites out there that you need to compete with.
I do like the layout, however, I would change your font from Times New Roman to something a little more smarter (Verdana or Tahoma).
On the Demo tagboard, I would reccomend making the actual box in which the posts are displayed bigger than it is, not a huge increase, just so that the form below it isn't bigger than the actual box.
In the members area, maybe have more than one page, instead of cramming it all onto the one page.
I'd also reccomend using cookies for the login, as it displays the username and password in the URL on the members area. It would make it easier if you stored a cookie and would mean to you could add features such as being able to delete posts directly from the tagboard (when your logged in as an admin, it recognises your cookie and you can delete posts directly on the tagboard).
Finally, if the website kicks off, your going to be swamped with bandwidth, I'd reccomend creating a preimuim service. (Auto refresh, custom smilies, ect)
All the best of luck with the website anyway, and I hope this review helps you out a little
