Automated testing, a tenet of the buzzword-compliant "XP" Extreme Programming, can be seductively dangerous though. Beware, you will write tests first, but they clearly won't encompass the problem....or worse, you will write tests later and will unknowingly write tests that all pass. A good test suite is very difficult to build, and the need for manual testing can never be eliminated.
Far too often in the software industry does an emphasis on automated testing and formal test organizations (i.e. "testers by occuptation") result in poor manual unit testing. A developer really has to understand all of the corner cases before manually unit testing, and to write effective unit tests, he has to be even sharper.
Anyhow, be warned -- automated testing is great stuff -- but it is not a substitute for the real thing. Your test cases passing doesn't mean there are no bugs!