Well we’re seeing a lot more MySQL customers these days.
— from a manager at a DBA outsourcing company
Purely anecdotal evidence, but I assume as databases like MySQL have proved themselves capable of doing a comparable job to Oracle, DB2 or SQL Server the cost issue has begun to make a difference. And if you’re a new business then a commercial database isn’t compelling. When more of the enterprise software vendors start supporting products with an option beyond Oracle, SQL Server, and DB2 adoption will increase. That took a while for Linux, but eventually everyone started to support at least RedHat for their enterprise software installs.
Could also be that companies hacking together a site in PHP who don’t know much about databases have to go looking for help as soon as they get large numbers of users.