I recently presented on one of my favorite topics at my local PHP user group, the South Florida PHP Users Group. The talk was meant to be an introduction to PHPUnit - I chose to dedicate some time to describing the need for automated testing. I think many PHP developers have a hard time seeing the benefit of unit testing. My main point was that developers already do tons of manual testing, so why waste all those precious hours when you could simply get the computer to handle it for you.
These are they slides, feel free to grab them from slideshare. I will gladly answer any questions people have.