From the official ComiXology Tumblr:
“We have introduced a new comiXology iPhone and iPad Comics App and are retiring the old one. iPhone and iPad users will now buy comics on comixology.com and download to the app. All your purchased books will be readable in the new app once you’ve downloaded [them.]”
Welp, I was excited about them teaming up with Amazon, but those feelings vanished in an instant this afternoon.
In the very same post they mention a new way for Android users to purchase comics in-app. I can only guess this means Amazon doesn't want to share a flat 30% of their sales with Apple, which is silly if true. If the overwhelmingly negative reactions I've seen on Twitter are any indication, their bottom line is about to hurt a lot worse than if they'd just stuck with the IAP model.
Good luck with that, Amazon.