Unity is a great tool for creating both 2D and 3D games on iOS. This tutorial blends a 2D HUD/UI on top of a 3D Fruit Ninja inspired game. You'll work with physics, textures and get some insight into the differences between the 2D and 3D behaviors of Unity. Try it out.
So, you're considering a multiplayer game? Knowing how to connect your players over a network is an obvious requirement, but doing it well is key to a good multiplayer game experience. This post examines various techniques for transmitting and syncing game state between players' devices during a multiplayer game and offers tips for avoiding common issues. It doesn't focus on iOS networking specifically but it does point out some key networking gotchas.
If you're thinking of writing low-level highly optimized graphics rendering on iOS then Metal is a good alternative to using Open GL ES directly. Not only is Metal easier to use and a more developer-friendly API, but it can also outperform Open GL ES in some cases. This post clearly explains the Metal rendering pipeline with some solid examples and points out the possibility of using Metal to harness your GPU's power for straight parallel processing.
This isn't a post about iOS game development per se, but it does offer a neat approach to representing data in a fashion that optimizes game AI. Shawn Hargreaves explains how the game Moto GP stored motorbike coordinates as they raced using a clever coordinate system that really improved the efficiency of their AI. You might be able to use this approach in your own games, especially if they involve lots of units or fast motion.