The documentation for NUnit states that a class with the SetUpFixture will run once before any tests in the namespace execute.
Our integration project has a separate namespace for the databases tests and has a class with SetUpFixture to create and drop the databases as required.
However we noticed with NCrunch alot of our other non database tests are running slowly now and after debugging I've found this to be that after running a single test from a different namespace, the SetUpFixture gets executed as well, even though no tests in that namespace are being run !