This allows you to create a single class that can be compiled in different projects to use unit tests in either VSTS or Nunit. From Microsoft's Enterprise Library January 2006:
Where NUNIT is a defined constant in your project file.
#if !NUNIT
using Microsoft.VisualStudio.TestTools.UnitTesting;
#else
using NUnit.Framework;
using TestClass = NUnit.Framework.TestFixtureAttribute;
using TestMethod = NUnit.Framework.TestAttribute;
using TestInitialize = NUnit.Framework.SetUpAttribute;
using TestCleanup = NUnit.Framework.TearDownAttribute;
#endif