In case you're not tuned into Sam Gentile, he just posted about a feature of  TestDriven.NET that I knew about, but never tried until now:

With the “Test With...Debugger” right menu option in TestDriven.NET, you can simply out your cursor on the unit test name you want to debug (in the code) and choose this option. The debugger will automatically start up in your unit test and you can set breakpoints.

This is great for doing legacy test harnesses.  I'm nearing the end of writing hundreds of tests to harness our existing business tier code here, and TestDriven.NET has proved invaluable.  I'm hoping to blog more about this process.  This experience has definitely paid off, I've uncovered countless bugs and dead code.  Best of all, I'll be able to begin refactoring stuff without worry of making breaking changes.  I hope I'm not sounding preachy, but if you're not doing TDD or at the very least unit testing your code, you'd be doing yourself and your successor a big big favor by learning about testing.  -Brendan