Couple of things I want to add on my observation on using MSTest vs NUnit. I always like to keep my deployment project and test project separate so that when I deploy my application there will be no test code sitting in production. By setting up a environment like this have a limitation when using NUnit. I might have some method private which if I want to test them using NUnit I can’t, since it can not find it. Only way I can test in NUnit is by exposing all the methods as public. On the other hand, with VS2010, I can right click on a private method and it creates test for me in the test project. So my class is clean and I am not exposing the helper methods as public. On this aspect, MSTest is awesome. This is a debatable . If you are creating API then you should be fine by exposing them public, but I do not like the idea to make methods public to support external tool. I like to keep the class clean. On the other hand, MSTest lacks some of the simple fundamental assertion NUnit have, like collection assertion (without looking for separate method) and able to use TestCase to capture multiple test case against single test to keep the test DRY.
I am still using MSTest and NUnit in different project. MSTest along with Gallio really good, on the other hand, NUnit/NSpec/Growl is so cool as well. I hope MSTest will come up to speed on NUnit so that I can stay on MSTest.
Unni's space 
Then you have designed your code to be fully testable and you need more “seams” created in order to test. This is the main benefit of TDD – better designed software. That problem “Only way I can test in NUnit is by exposing all the methods as public” is what we would call a code smell. Time to refactor your design.
Excellent point. Did not think that way, when you think from TDD perspective, we shouldn’t limit to that. Thanks for pointing out.