There are two main types of cell phone system: CDMA and GSM. The Global System for Mobile Communications (GSM) was created in France throughout the 80's and the EU endorsed it as their official system, which caused it to spread globally. Meanwhile, across the pond, we were sitting on our little keisters and our brick analog phones and then a company called Quallcomm decided to do something six years after GSM had publically been out and they created a popular version of CDMA. CDMA is currently used by Sprint and Verizon (and I think a few Canadian carriers) and is pretty much only existent here in America. GSM is present in 78% of the world's markets.
With that said, GSM phones will not work on CDMA networks and vice versa. If Apple does make a phone, I think it would be GSM in order to capture most of the international market as well as the US. CDMA is very limited because it is not used anywhere besides a few carriers here in America.