I've already written about how much Junk Jack helped me out on my multiple flights back from Europe recently, and the other iOS game that really kept me moving from airport to airport on that trip was 100 Rogues, a roguelike RPG that arrived on the App Store a while ago. It just recently got an update with yet another new class, but if you like the turn-based exploration and combat of roguelike dungeon crawlers at all, this is the best example of the genre I've seen on iOS.
Controls are simple -- just tap to move or attack, depending on the context. The graphics are simple and colorful, but get all the information across that you need. And I liked the game's depth -- not only can you of course kill monsters and collect loot, but there are some fun elements in there as well, like ranged combat, thrown attacks, and even things like earning bonuses on weapons after you use them for a while.
100 Rogues is a great game, and delivers exactly what you want from a roguelike: straightforward turn-based RPG action. It's on the App Store now, in a universal version, for just 99 cents.