You're not going to get 65k queries per second out of an RDBMS if any of those queries have to do JOINs, and you're not going to put Varnish in front of it to speed it up even further. You can do that with memcached and CouchDB, for example.

I'm by no means suggesting that there's anything wrong with JOIN, or that every persistent data storage needs to do what eBay or MySpace or Amazon.com does. Yet by the time your problem is that your database is a bottleneck, slapping something on top of the database doesn't seem like the right approach.

(Similarly, I have the strong belief that if ActiveRecord could run on something other than an RDBMS, everyone would be a lot happier.)