.Net: Best Way To Organize Unit Tests

I am personally used to creating a test project per every single project for which unit tests are being written, and keeping them in the same solution, but it seems to me that some people do not believe that is the best (notable observations include that it bogs down the solution when you get to over 100+ projects, and that it can be difficult to separate the unit test binaries from the delivered product) Some of the other concepts I have seen include creating a separate solution for all the unit test projects, and some even suggest putting all the test cases in a single project (to me, that sounds like a bad idea) either inside the regular solution it is associated with, or  in its own solution.

What is everyone's take on this?

That's because you're looking at it the wrong way. When you see a cyclone from the top, you are seeing where it is releasing the air that it sucks up. Thus, the direction of reference is traveling outward from the center along the spiral, not inward.