You can use C++ on the iPhone, wrapped in ObjC for the API calls. See http://iPhoneDeveloperTips.com/cpp/c-on-iphone-part-1.html.

As far as using those features, it's fairly easy to do so using the iPhone SDK. I had no exposure to any other platforms though.

I'd rather not port code, but honestly, 80% of the project will be new code from me, not old stuff, and I think ObjC is a nice language. I'm most concerned about the speed of access to and ease of use of the camera and sensor interfaces.

It might be a good idea for you to look at currently open-source iPhone apps and check out the source code. Specially those that interact with the camera and sensors. Here's a few links I found for you:

http://opensourceiphonesoftware.com/

http://sunsetlakesoftware.com/2008/10/02/molecules-source-co...

http://petersteinberger.com/2009/05/open-source-iphone-apps/

And one that interacts with the camera:

http://code.google.com/p/zxing/