Is there an online Q/A forum for the philosophy of mathematical practice?

I think it is not an accident that philosophy.stackexchange is so very uninspiring (to put it tactfully!). Philosophy really isn't the sort of enquiry which in general lends itself very well to the stack exchange format, which is most appropriate for inviting short sharp technical answers to focused questions, and not so apt for exploring conceptually complex, ramifying, debates. And what applies to philosophy generally applies to philosophy of mathematics in particular.

Still, that's no reason not to pose specific questions here or at mathoverflow.net when broadly philosophical/conceptual questions and technical concerns do entwine, and you might get some really good responses (think e.g. of some of Joel David Hamkins's contributions over at mathoverflow).