People have been debating for a while now on how gps enabled phones are going to be the future of the gps navigation industry. However first reviews of gps enabled phones have been bad. People have been complaining about not been able to lock onto a gps signal even if they are outside and that the monthly charges for the gps service are too high. These are probably a few of the reasons why Apple choose not to include gps in the iPhone.
So keep buying the stand alone gps systems. It seems as though it will be a while before cell phones are able to catch up to the quality and service that stand alone systems provide today.