In a new post to his blog Marco Tabini offers some suggestions on unit testing - not really a tutorial on how to it, more of an "easy way in" to introducing it to your development process.
Stopping development for weeks while you figure out how to add unit tests to cover your entire codebase is simply something that cannot be done (at least, not if you want to keep your job), no matter what future benefits it might bring. The good news is, adding unit testing to your existing project only takes five minutes - which is pretty much how long it takes to get a unit testing framework installed. That's it. Move on.
He puts the emphasis on unit testing to manage change in a code base, not so much to ensure that the current application runs as it should (not initially at least). He's found them most useful in bugfixing, refactoring and when adding new functionality. Current tests (and even tests written in TDD) can help with all of these. He includes reminders that if the tests aren't written well, they're useless and that once you've started testing, it needs to be continuous, even if they're not perfect.