This is a fascinating post from GPS-practice-and-fun.com which details the increasing blending of GSM technology with GPS to improve location accuracy. GSM, a cellular technology used worldwide, gives GSM enabled GPS devices a second network with which to perform location pings. If you device cannot get a precise GPS fix, it can cross reference available GSM signals to further triangulate your location. Very cool! Check out the original post for more detailed info.
GPS enabled devices are often less accurate than standalone GPS receivers!