Let me clarify a little bit. We are unlikely to be able to dedicate a lot of resources to a Mac version, since we're an indie with limited resources, as noted. However, even though it is a smaller market by far, there are also fewer game titles available for the Mac, so it may still make sense for us.
The reason we are at least open to the idea, is that the frontend engine we use, Unity, also runs on a Mac (in fact, it even runs in a browser). What remains to be seen (after we have made the darned game in the first place!) is what kind of challenges it would take to port the .net backend to Mono. If that could work without major pain, it is a possibility. The same applies to Linux, assuming Unity ever runs on it.
Gentlemen, I hope this thread doesn't evolve into a Windows vs Mac vs Linux debate. :)