MbUnit supports different flavors of parameterized unit tests: RowTest, CombinatorialTest, etc... If you are already using those features, it would be very easy for you to 'pexify' them:

namespace MyTest
{
     using MbUnit.Framework;
     using Microsoft.Pex.Framework;

     [TestFixture, PexClass] 
     public partial class MyTestFixture 
     {
         [RowTest]
         [Row("a", "b")]
         [PexTest]

         public void Test(string a, string b)
         {
             ...
         }


     } 
}

Isn't this nice? :)

Some little notes:

• 'partial' is helpfull to emit the generated unit test in the same class... but not the same file. Pex also support another mode where partial is not required.
• the Pex attributes do not 'interfere' with the MbUnit ones. Your unit tests will still run exactly the same with MbUnit.