Jakob got out a cool checkin policy after a question on the microsoft forums about this. The policy ensures that you have run a code coverage, and that the coverage percentage is above a certain set limit. See Jakob's blog for a description http://geekswithblogs.net/jakob/archive/2009/02/23/writing-a-code-coverage-checkin-policy.aspx
The code is available on codeplex, see here http://www.codeplex.com/TFSCCCheckinPolicy.