Black Library's site posted up the preorder for Digital copies of the Farsight Enclave and the Main Rules.
Disappointingly, the second paragraph of the product description begins with "Also included are new missions and rules to use with your Tau Empire collection..." I might be reading a bit too much into it, but the way the new rules are being referenced as fairly peripheral is hardly encouraging for those of us holding out the hope that some supplements at least would get a bit more in depth treatment that the 2-page Iyanden Supplement.
The digital Rules release comes as a bit more of a surprise, and while it's a nice idea, I'm not sure I'll be willing to shell out for the non-interactive ebook version, and don't have an option to purchase the Apple version. Not to mention, something about buying a book that expensive for a second (or possibly third for those who also bought Dark Vengeance just for the pocket rules book) copy.
Edit/Addendum: Kirby at 3++ posted up info from the iTunes store, including the sneak peek images: http://www.3plusplus.net/2013/07/farsight-supplement-tau-codex/
All that said, what do you think? Are you considering one or both?