Luke Francl of Tumblon has a nice summary with backing research showing how unit testing is only part of the QA story.
Its important to realise that developers by nature will only test the happy path; hence its likely that those diabolical edge cases will remain untested by a developer.
Another important factor is that most bugs come from missing features or requirements; and its impossible to unit test what isn’t there.
See more at http://railspikes.com/2008/7/11/testing-is-overrated; specifically Luke’s create Venn diagram showing how different types of testing (unit, user, code review etc) uncover different types of defects.
This isn’t to say that unit testing isn’t worthwhile; it just frees your testers up to concentrate on the unexpected, non-logical things that users are sure to try use your software for