You know c++ and OpenGL and have a Mac, so you're pretty much set. Just download Xcode and the latest iOS SDK which are both free. Then build one of the OpenGL samples and you can go from there. Just make sure that all the game and graphics code you add from there on out is in c++ and separated from the Objective-C code in the sample. That way you can pretty much ignore the ObjC stuff and work in what you know, which will be good platform-independent c++ code.
Edit: Oops, I misread the part about not having a Mac. In that case you could get a Mac or go with something like Unity.