Not at all! You can buy a few C++ game engines (e.g. Torque) and port them. I have Torque running on Linux, Solaris, Mac, & Windows. Considering it's normal posix underneath, you just have to redo some plumbing.
The iPhone is POSIX, but not to you as the developer. Porting Torque to the iPhone will have to be done by GarageGames, not by you - even if you pull it off it will certainly be violating a laundry list of App Store rules.
Well, for one thing, you have no file system - and that's just first among the list of POSIX-y things that Apple internally has access to, but you do not as a 3rd-party.
The amount of plumbing to be ripped out is pretty extreme given that you don't have access to large bits of the OS.
Not to mention Torque is not written with OpenGL ES in mind - so you'd have to rewrite not only plumbing, but some core graphics code.
To make it an Apple-compliant app, porting Torque (or most desktop engines for that matter) is simply not feasible/worthwhile.
