Agreed, we have massive sites serving millions of requests a day using Open Source relational databases and yet it seems everyone wants to use NoSQL because it's the hip new thing.
Naturally I start thinking of this: http://xtranormal.com/watch/6995033
A million requests per day translates to 11.5 requests per second. That's a pretty trivial amount of traffic. A massive site like Facebook is probably serving about 4 orders-of-magnitude more requests that that.