Over on Comixology's blog they've posted that Apple has delayed several of their day and date releases due to them still going through the approval process and that subsequently not all of the comics that they usually have to offer digitally will be available today. They do recommend buying it from their website and then downloading it to your Apple device to short cut the system. This is the same thing you'll need to do if you want to read Saga #12 digitally as well.
Frankly I find this move from Apple to be very suspect. Comics Beat took an in depth look at Apple's content approval system and has several pages of the Saga issue in which The Will heads to Sextillion which has twice as much sexual content and nudity on the page and was approved flawlessly. Perhaps Apple having this rubbed in their face decided that they should pay closer attention?
Comics Beat's article also has a quote from Sam Humphries where he encourages people buy their comics directly from Comixology's site to begin with so that less money goes to Apple from the sales. I have to support this move (even though Comixology is practically a monopoly) because frankly who the hell wants Apple to control comic content?