<p>If you were to mandate a minimum percentage code-coverage for unit tests, perhaps even as a requirement for committing to a repository, what would it be?</p>

<p>Please explain how you arrived at your answer (since if all you did was pick a number, then I could have done that all by myself ;)</p>

85% would be a good starting place for checkin criteria.
I'd probably chose a variety of higher bars for shipping criteria - depending on the criticality of the subsystems/components being tested.