Updated with comment from Square Enix, who promises to remove the ban on jailbroken devices.
Deus Ex: The Fall is getting a fairly warm reception, but if you've modified your iOS device, you may want to resist buying the game. According to some early adopters, the game doesn't let you use guns if your device is jailbroken.
The restriction was caught by a user on Reddit (via Kotaku) and subsequently backed up by reviews in the App Store. When you hit the part of the tutorial that involves firing your weapon, an error message tells you, "We are sorry but you can't fire on jailbroken devices." Though you could conceivably stealth your way through without a weapon, failing to complete the tutorial by tranquilizing the guards may mean you can't progress through the rest of the game.
Jailbreaking your iPhone is perfectly legal, but voids your Apple warranty. It's a common practice among users who want to run homebrew apps and customize their iOS devices beyond the boundaries usually available.
Needless to say, iOS users who bought the game legitimately at its relatively high $6.99 price tag are upset. "I understand protecting the app against cracking/pirating, but preventing paying customers from playing on a device they own and legally modified is ridiculous," one user review read.
Update: Square Enix has issued the following comment to Joystiq, promising an update that removes the ban on jailbroken devices:
We have not been clear in our communication earlier this week when we launched Deus Ex: The Fall. We did not state clearly that the game would not support jailbroken devices and so we will be switching this off via an update, so that all the supported iOS devices will be able to play the game in the near future. We feel it's the right thing to do in this situation and apologize for any inconvenience this may have caused.