Goran has pointed out a plugin for Eclipse that can help those out there wanting to adopt Test-Driven Development into their process - MakeGood (screenshot here).

MakeGood provides a test runner to run unit tests on Eclipse PDT. MakeGood strongly supports Test Driven Development (TDD) by various features.

Features in the plugin include:

• Runs all tests when a file is saved.
• Debugs a test.
• Reruns the last test.
• Can use the system include path when running tests.

It supports both PHPUnit and SimpleTest so you're covered either way. It also supports the Zend Debugger and Xdebug for help with your debugging needs.