I have heard of the upcoming "Cryptography" stackexchange but I'm not sure when it's going to be in public beta. I'm wondering if I could ask a cryptography question here without being "off-topic".
If you are asking about the mathematics behind cryptography, absolutely. This usually includes theory of many cryptosystems (especially modern ones), and cryptanalysis, as well as problems of complexity of algorithms (a bit more borderline, but still okay).
On the other hand, questions about actual implementation of cryptosystems, technical attacks based on implementation, or steganography, are very likely to be off-topic.
(There are some exceptions, but they are far an between; for example, there are some attacks on certain public key cryptosystems based on the idea of bit errors or incorrectly chosen moduli/secret keys, and the actual attacks are based on mathematics; those are fine. But for example attacks that rely on measuring the amount of work the processor performs, etc., are not).
Depends on what you mean by a cryptography question. Generally speaking, a question about the conceptual details of an algorithm (e.g. RSA) are probably fine. A question about implementation details, on the other hand, is not really a math question and should go on, say, StackOverflow.