This isn’t much of an article in and of itself, but is a good jumping off point, with some good links at the bottom - http://bit.ly/ahUMF2
To quote from the article (entitled “MySQL + Memcached: End of an Era?”):
“LinkedIn has moved on with their Project Voldemort. Amazon went there a while ago. Digg declared their entrance into a new era in a post on their blog titled Looking to the future with Cassandra. Twitter has also declared their move in the article Cassandra @ Twitter: An Interview with Ryan King.”
Very, very few sites reach the heady heights of these sites (watch this video for some pocket stats, from which the slide below is reproduced), so I guess most people will be happy with MySQL for a while yet, but interesting nonetheless.