Reading this post by Sean reminded me of when I first started using the trunk's BaseControllerTest.  One nifty little property on the base test controller's mock Response object is RedirectedTo.  So you can do something like this:

   1: // some call on the controller that should do the redirect

   2: Assert.AreEqual("/productSearch/performSearch.rails", Response.RedirectedTo);

In previous MonoRail projects, I used the very nice base class from the guys at Eleutian and it worked great.  But I've been using the trunk's BaseControllerTest exclusively on my current project since it started a few months ago and it rocks as well.  Nice to have it "out of the box" now.