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/