Since I started writing this I got some information about current problems with MySQL. Rumour has it that MySQL is working on this. http://tweakers.net/ has been doing a lot of testing of running MySQL and PostgreSQL (with different hardware). Below are links to some articles they have written about this. Take a look at the graphs there. It looks to me that there indeed is some problems running MySQL with many concurrent users on some hardware (if I do not misunderstand something there):
