Location of unit tests.

I had a short conversation at Alt.Net Canada about the location of unit tests. I personally tend towards a distinct unit test project. But, I deal with mostly commercial, off-the-shelf (COTS) projects where I simply can't ship code like that. I also don't want to wire-off the unit test via #if because I would then be shipping something different than that which was tested.

From an enterprise application point of view, this is different. I would have no problem including the unit tests within their respective project as production code