Chase and I talked about this briefly on Thursday. Though Necrons are long rumored to be the next codex (following the human / xenos pattern), Chase has heard that the Tau may be next. Reason being, they would be much simpler to update. I sincerely hope this is not the case, as Necrons likely would have to go at least an entire year before a codex update. Also, Tau don't need an update. The right lists remain highly competitive.
Does anyone recall when GW officially announced the GK codex?