I suppose by now you have all downloaded the Visual Studio 2010 CTP. (Well, maybe not.)
But if you have and you are looking for a great unit testing framework for your first experiments with .Net Framework 4.0, I have published an early preview of Gallio and MbUnit v3 designed to work with the CTP.
Out of the box, it supports the VSTS test runner so you can run your tests using the Visual Studio Test View. Just be sure to create a new test project using the MbUnit v3 Test Project template.
Try it out!