I have been following the trend involving "NoSQL" technologies for some time now. Much like arguing that there is one true best language for developing software, I'll openly acknowledge there is no single true best data store technology. It is nice to see some very well thought out counter points appearing debunking the anti-RDBMS FUD. The following are particularly good ones:

Dennis Forbes' article "Fighting The NoSQL Mindset, Though This Isn't an anti-NoSQL Piece" steps through a rough benchmark scenario.  Even though Forbes' system is less than top of the line, he does a good job showing how a RDBMS can be tuned to yield much higher performance and notes that MySQL is well known for having the scaling issues sites like Digg, Reddit, and others faced.

Also good, Paul Ramsey's NoNoSQL is worth a read.