Code Coverage

http://www.bullseye.com/minimum.html
Minimum Acceptable Code Coverage

Summary
Code coverage of 70-80% is a reasonable goal for system test of most projects with most coverage metrics. Use a higher goal for projects specifically organized for high testability or that have high failure costs. Minimum code coverage for unit testing can be 10-20% higher than for system testing.
References
Efficient use of code coverage in large-scale software development 2003
Code coverage, what does it mean in terms of quality? 2001
An Empirical Study of the Branch Coverage of Different Fault Classes 1994
Coverage measurement experience during function test 1993
Lessons learned on the road to 100% code coverage
How to Misuse Code Coverage
Comparing the Effectiveness of Software Testing Strategies
DO-178B, Software Considerations in Airborne Systems and Equipment Certification