If my code contains a known defect which should be fixed, but isn't yet, and won't be fixed for the current release, and might not be fixed in the forseeable future, should there be a failing unit test for that bug in the test suite? If I add the unit test, it will (obviously) fail, and getting used to having failing tests seems like a bad idea. On the other hand, if it is a known defect, and there is a known failing case, it seems odd to keep it out of the test suite, as it should at some point be fixed, and the test is already available. 
The answer is yes, you should write them and you should run them.

Your testing framework needs a category of "known failing tests" and you should mark these tests as falling into that category. How you do that depends on the framework.

Curiously, a failing test that suddenly passes can be just as interesting as a passing test that unexpectedly fails.
