google uses mysql but they have their own patchs for mysql check it in google code so far till now google is using mysql ,but after oracle bought sun which owns mysql who knows what will happen to mysql. Mysql has been forked into many projects after sun bought it in 2006 . who knows google might start their own forked version of mysql if oracle is gonna kill it in the future.
Several years ago one of the popular computer magazines had a write up about what operating systems and database servers the BIG sites were using. This was back in 2003 or 2004. The article looked at sites like msn, yahoo, google. The author of the article said google used a heavily modified version of Unix and mysql. There was a mention about a Unix core and mysql, but that was about it. The engineers has modified the system so much, it was more custom then anything else. What I got from the article, when google first started it used unix and mysql. But over the years the developers changed so much that is was now more like a custom OS and database.