It’s not a topic that gets raised to often, but Richard Fennel gives a very thorough description on how to enable cross-version running of Ivonna and Isolator.
Richard mentions the Typemock.Integration assembly. This assembly is deployed as part of Isolator and is designed to allow Isolator to integrate with add-ins, such as Ivonna (which is great for ASP.Net unit testing). As opposed to Typemock.DLL it does not reside in the GAC, rather in the application folder.
Great post, Richard!