The Oolong Engine is written in C++ with some help from Objective-C. It will help you to create new games and port existing games to the iPhone and the iPod touch. Here is its feature list:

OpenGL ES 1.1 and OpenGL 2.0 (only on iPhone 3GS) support
Math library that supports floating-point calculations with an interface very similar to the D3D math library
Support for numerous texture formats including the PowerVR 2-bit, 4-bit and normal map compression formats
Support for PowerVR's POD (Scene and Meshes), .3DS and .blend file formats
Touch screen support
Accelerometer support
Text rendering to support a basic UI
Timing: several functions that can replace rdstc, QueryPerformance etc.
Profiler: industry proven in-game profiler
Resources streaming system
Bullet SDK support (for 3D Physics)
Audio engine with OpenAL support
Networking with the ENet library