Re: bbpress and memcache?
“[WordPress.com] uses several dozen queries per page load.”
Thanks to memcache the average blog front page on WordPress.com uses only a handful of queries, sometimes as few as one.
“It absolutely cannot just rely on the mysql cache, even with dedicated mysql servers.”
From memory, WordPress.com doesn’t use MySQL query cache at all as it would need to be invalidated many times every second. We also would need to invalidate all slaves as well as the masters.
I think it’s important to say that you should only setup and enable any caching if you actually need it. Most people don’t and shouldn’t, all you do is add another potential point of failure into your site.