The cells are in dire need of padding :P Probably one of my biggest design pet-peeves is when there isn't enough padding around text and it hugs too closely to borders/other objects.
The footer thingy on the page, for example. Doesn't that drive anyone else nuts? Maybe I have a problem...
Otherwise, good stuff. You should post a query count and page exec time or something, too. Load times in themselves should be much faster without the kilobytes of codebloat that current forums use, but optimzing queries and such would make it even more popular.
Another thing: Is it object-oriented? I'd imagine it would be, but if not, would be a good way to go seeing as people love modding forum software and doing that with procedural PHP can get ugly.
Good stuff though. Am planning to write a CSS/XHTML strict forum myself. Forums themselves are fairly simple, but packing them with all the features vBulletin or XMB have is quite the task. A CSS-styleable forum would definately be a popular product.