Screenshot from PC version of Cube
Cube is an open source multiplayer and singleplayer first person shooter game built on an entirely new and very unconventional engine. Cube is a landscape-style engine that pretends to be an indoor FPS engine, which combines very high precision dynamic occlusion culling with a form of geometric mipmapping on the whole world for dynamic LOD for configurable fps & graphic detail on most machines. Uses OpenGL & SDL.
According to the developer "everything" has been ported "including networking and editing - though there is no practical interface to use it". That being said, the developer says this version is intended as a technology demo, so it doesn't appear that there will be multiplayer servers available. Cube will be available for free and has been pending review by Apple for a number of weeks already.
It appears the developer also has hopes to port Sauerbraten (Cube 2) to the iPhone at some point in the future. There's been some anticipation of a proper iPhone shooter, with earlier previews of IUGO's upcoming game Re-Volt.