I recently got this email from a reader on my site and since I haven't posted for a while, I thought it might be a good discussion:
I've been reading your site and had a question that you might have good insight for.
I'm working on a high-traffic website that includes forums. All the code is custom PHP, including the forums, simply because the PHP BB software out there doesn't scale as large and as well as I wanted. I've implemented heavy memcached usage, distributed databases, etc. to handle any level of growth we may hit.