I decided that my client would be overwhelmed by a lot of bells and whistles in a message board like phpBB, so I started from scratch. So far everything has been working really well, apart from a little issue with quotes in replies.
When the user clicks Quote, they see this (using tinyMCE as my WYSIWYG): alt text http://tapp-essexvfd.org/images/post_capture1.jpg
This is a blockquote styled with CSS, works great as long as it is by itself. But clearly this would pose a problem if a user were quoting a post that had a quote already (nested blockquotes).
One solution I considered was changing the blockquote tags to spans before they are entered into the editor, hoping that someone here might be able to provide a simple solution. I did consider using a span instead of blockquote but I like that the user has the ability to click the blockquote button in tinyMCE.