Hi Arnulf,
That is actually a very good question.
Unlike many other mocking framework out there (including Rhino).
The Isolator has no inherent limitation and can mock virtually anything.
Specifically, the Isolator can mock concrete classes as well as Interfaces, and it can mock static methods as well as instance methods.
In short as Gil mentioned we already support most of the new features Microsoft released as part of the .NET 3.5 framework including LINQ. :)
:idea: There are several good articles out there on how to Mock LINQ using the Isolator here are two of them:
Mocking LINQ Queries, Extension methods and Anonymous Types
How To Unit Test - Linq to SQL and Mocking