It's hard to take the question seriously, given that Google, Travelocity, etc. care deeply about their businesses, and entrust them to open-source databases. And yet the question still comes up, as it did at eWeek recently.
eWeek asked Kevin Closson, chief software architect for PolyServe-HP, who answered 'Yes,' open-source databases (like MySQL and Postgres) are serious alternatives to Oracle et al, but that it depends on the application:
[I]n modern multi-tier applications you have a lot of intelligence in the application server tier and even in the browser on the user's desktop. If the database server goes down, all is not lost, because you have persistence in the front-end and the middle tier. After a certain delay you will be able to reconnect and finish your transaction.
In that situation it may not make sense any more to spend $40,000 or even $60,000 per license for a database like Oracle 10g when MySQL or Postgres or Ingres could do the job.
Interesting. The application stack gets some credit for taking the load off the database. But, again, when you look at the industry's technology leaders and tomorrow's leading retailers/enterprises (Amazon, Google, etc.), you see MySQL and Postgres everywhere. What do they know that old-guard enterprises don't? Well, technology, for one. But it will trickle down.
An enterprise might not want to run everything on MySQL or Postgres, of course, but the reasons for defaulting to Oracle are slipping...and fast. Especially when you consider that you can get drop-in Oracle compatibility with EnterpriseDB's PostgreSQL distribution.
You could use that savings to buy lots of things, like sending flowers to Larry Ellison to thank him for all the good times. Surely there were good times...yes?