There's a nice article by Jennifer Mears of TechWorld on why Union Bank of California has switched from AIX to Red Hat Linux and the LAMP stack (Linux / Apache / MySQL / PHP). This article goes in greater depth than the earlier reports interviewing CTO Mok Choe on the reason for the move, the applications etc. Union Bank is one of the 25 top banks in the United States and while a lot of firms in the financial service sector are using the LAMP stack and JBoss, few have been willing to go on the record. It's smart marketing on their part. Not only does it show that they are not locked into closed source legacy software, it's makes Union Bank a more interesting place for IT staff who want to keep their skills up to date.
Choe took a gradual well thought out approach to adopting open source, ensuring that they were successful with smaller pilot projects before going into widespread use. He also made sure that there was a full return on investment (ROI) analysis in place as well as internal audits validating their use of open source. The key driver on their widespread adoption was a desire to lower the transaction costs of the bank by moving towards a scale-out architecture based on open source. Or as Choe puts it:
"We were faced with having to do an upgrade of our Web infrastructure last year, which gave us an opportunity to really look at our architectural direction. We decided to head in the direction of horizontal scaling using commodity hardware and open source tools. There are multiple drivers behind this thing. Cost is clearly one of them. First and foremost always is the reliability and performance. The second thing is we wanted to be able to scale much easier than we do today... We wanted to be able to rapidly manage capacity by adding or subtracting commodity hardware."