In this new post to DZone.com, Giorgio Sironi talks about a method of testing that's non-invasive when you're in a chicken-and-egg kind of situation:
The legacy code dilemma, however, is always present: you can't refactor the code before putting up some tests on it to avoid regressions and putting your application offline. At the same time, you can't easily unit test the code until some refactoring is introduced.
He suggests making copies of some files to allow you to make small changes where needed to "mock" resources in the application to prevent it from accessing the actual data sources. His method fakes HTTP requests to the script and uses the copy of the script as an internal resource. Don't worry, code is included showing how its done - a basic "ForumPosting" class that includes the needed file and wraps the output in a buffer.