With NUnit, if you add Debug.Print statements in your tests they appear in the test output. (At least they do in the ReSharper unit test window.)
When using a NAnt build
Console.WriteLine() should be preserved (it is for my version of NUnit at least).
Console.WriteLine()
That said, consider - each time you want to add some text output - how you could turn it into an assertion with a message. Your tests will get much better.