In a recent post from the AppBackr blog they share a solution they've found for running unit tests automatically for lazy coders using a combination of PHPUnit, Watchr and Growl notifications (OS X-only, obviously).

Now you could run [your] tests manually after each change by typing 'phpunit ', but we were looking for something a little more comfortable. We want to decrease the transactional cost of running the tests as much as possible to make the lives of our engineers more pleasant. Making testing easy is the only way the engineers will run the tests frequently.

Instead they opted for Watchr, a tool that keeps an eye on a list of files looking for a change. When one happens, a process is fired off - in this case a PHPUnit run. To make it even easier to see feedback, they integrated it with Growl to show pass or fail results. He includes the source for his Watchr script to help you get started.