Here is a schematic for a USB interfaced Geiger Counter;

http://www.sparkfun.com/datasheets/Sensors/Geiger_Counter-v14SMD.pdf

Or you just buy one here; then mount into a plastic box leaving a hole for the GM tube window.

or you could purchase one of these;

http://www.blackcatsystems.com/GM/products/GM10GeigerCounter.html

Both of these could be connected to an iPhone, even the iPhone App wouldn't have to be modified to greatly as is it is basically a pulse per second counting routine.

In more harsh radioactive environments the iPhone probably wouldn't stand up to the radiation though.