I have recently added a feature that allows the code coverage support built into Visual Studio 2005 Team Suite, Developer or Tester to be accessed using TestDriven.NET. Simply use the new 'Test With... Coverage' button you will find on most of TestDriven.NET's context menus. This will execute a subset of your unit tests with code coverage enabled. You can use it with NUnit, VSTT (Visual Studio Team Test), MbUnit, Ad-Hoc or any other supported test type.
I have created a video showing code coverage being used on the 'Money' unit testing sample that comes with NUnit. As you will probably notice I'm pretty new to using code coverage on unit tests. My instinct when working with the sample was to comment out and make failing tests for any of the un-covered code. I'd be interested to hear from anyone who has successfully incorporated the use of code coverage into their "Red, Green, Refactor" routine.
Code Coverage support is the first of many features that will be included in a commercial TestDriven.NET Team Edition. This however won't be the death of FREE versions of TestDriven.NET. Expect more news in the next day or two.
You can find the latest build here or on FolderShare.
P.S. Let me know if there any other code coverage tools you would like to see supported.