Designing a good web based message board for light use isn't overly complex. The real issues arrise from getting one that runs affectively under extremely high loads (exceess of 500,000 page views per day).
My experience so far is that this is alot tougher then it sounds.
If anyone knows of any articles or books that cater to performance tuning tips for php, please let me know. 🙂 I'm in the process of gearing up for my 3rd board implementation using php and SQL Server on the backend.