Yes you are - firstly, forget about unit testing as a reason to design your code around the unit test tools, it is never a good idea to bend your code design to fit an artificial constraint. If your tools force you to do this, get better tools (eg Microsoft Fakes/Moles that allow you many more options for creating mock objects).
For example, would you split your classes up into only-public methods just because the test tools do not work with private methods? (I know the prevailing wisdom is to pretend you don't need to test private methods, but I feel this is a reaction to the difficulty in doing so with current tools, not a genuine reaction to not needing to test privates).,
All in all it comes down to what kind of TDDer you are - the "mockist" as Fowler describes them, need to change the code to suit the tools they use, whereas the "classical" testers create tests that are more integration in nature (ie test the class as a unit, not each method) so there is less need for interfaces, especially if you use the tools that can mock concrete classes.