Retroactively writing unit tests for legacy code can very often NOT be worth it. Stick with functional tests, and automate them.
Then what we've done is have the guideline that any bug fixes (or new features) must be accompanied by unit tests at least testing the fix. That way you get the project at least going in the right direction.
And I have to agree with Jon Skeet (how could I not?) in recommending "Working Effectively With Legacy Code", it really was a helpful skim/read.
