Can I create a wrapper around NUnit, MbUnit, xUnit or other testing framework?

How can I create a wrapper around a testing framework? We still doesn't know which testing framework are going to use, but I need to start writing unit tests. With this question I want to know how can I switch from NUnit to mbUnit, xUnit or even MSTest.

You could create a wrapper - but I think you can utilise your time much better. I'd say pick the simplest one (My personal favourite would be the war-horse NUnit) that fits your needs - the newer frameworks add functionality that help you write more complex test fixtures.
However I value simplicity over "flexibility". In the future, if you find yourself wanting that "cool feature X in Y testing framework", you could either write that particular test fixture using Y. (you could also migrate the entire test fixture to use Y for consistency - but time is always scarce) - Switch between 2 unit testing framework is usually monotonous work (rename attributes) however some might be more work (disclaimer: no flying time with MbUnit)

Your comment however worries me a bit.

1. Why is the customer deciding the testing framework that you'd use for development - it should be a choice of the development team. The customer wouldn't want you to define product requirements - would he/she ? The quality of tests doesn't depend on the framework used so I don't see how this affects the customer.