In response to this older post Ralph Schindler has posted some of his own thoughts on database abstraction layers - they must live!

Interestingly, I can put the vocal proponents of each side of the argument in one of two boxes: a programmer guy box, or a database guy box. For some unknown reason though, they never seem to see eye to eye. [...] Generally speaking of database driven projects, I feel like planning to use a specific vendor up front, knowing its pro's and con's, and tailoring an application to the chosen database's strengths can only help in the long run. Also, I feel that building a database model first before any code, offers many performance and scalability advantages than does code first development.

He notes that, while he agrees on the problems with the "switching databases at any time" problems the other author brings up, he points out that the abstraction layers do have their place. He includes an example of an abstraction layer with the Zend_Db object of the Zend Framework. His sample code shows how it can be used to simplify your interaction with your database.