Q: Are open-source databases like PostgreSQL, Ingres and MySQL becoming serious alternatives to Oracle for enterprise applications?
A: Yes, more and more, depending on the application.
Oracle became the leading database in the 1990s because it ran better on high-end SMP Unix servers. But in those days most applications were still just dumb terminals talking to the big Unix box. So the database software had to be very sophisticated to perform well.
But in modern multi-tier applications you have a lot of intelligence in the application server tier and even in the browser on the users 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. They might not be good enough for a 911 emergency call center.
But if your application is a Web store that sells widgets, they might be fine. Given the high MTBF [mean time between failure] on the servers and disks that are available now, you might only get one or two unplanned outages per year, and for many small or mid-size businesses, that is perfectly acceptable.
But today I see even high-end IT shops asking these questions, too.