It seems that the Necron codex is no longer available. While not confirmed everywhere, it is at least gone on the US and UK Games Workshop sites. While product vanishing from stock on the site is not generally a "they must be releasing a new one", it is giving us a general picture alongside all the other rumors, that Necrons are indeed on their way. Soon.
It also appears a few other models are also no longer listed, but I have not had the chance to confirm it yet.