>Is there any way to get round this?  Do I have to rename all of the
>logic/control methods (ie something myAnd: [someblock])?

I prefer #and3;, myself, but i think this is certainly your easiest solution.

You might well consider this to be a good design decision as well, 
thus distinguishing three-value operations from two-value operations 
with type errors.

It will also keep the rest of the code in your system behaving well, and fast.