There is already a method to do this, GetArgumentsForCallsMadeOn(), although the syntax is not quite as neat as your version.

E.g.:

[Fact]
public void Can_check_argument_with_Rhino_Mocks() {
var testMock = MockRepository.GenerateMock();
testMock.DoSomething(10, “test”);
string secondArgOfFirstCall = testMock
.GetArgumentsForCallsMadeOn(x => x.DoSomething(0, null))
.First()
[1] as string;
Assert.Equal(“test”, secondArgOfFirstCall);
}

public interface IAmJustForTesting {
void DoSomething(int a, string b);
}

Cheers,
David