Matt Harrah recently wrote about his new toy: Project PEA. This thing is like test coverage reporting with a magnifying glass. Matt, Jeff Richley and I have had conversations in the past about the topic and the way test coverage reporting tools can give you a false sense of security.
The idea is to find the spots in your test coverage that may be 'accidental' having been buried beneath too many layers to be effective. (hence the name of the project.)
If you work in an environment with test coverage requirements, you may want to take a closer look at Project PEA.