The thing with the GPS..
It's all about update-speed of the location.. if you just occasionally need to know where you are (to find - for instance - a nearby coffeeshop) you can perfectly use triangulation of the GSM-antennae (at least in europe that service has been around for years.. tie that in with googlemaps and you've got quite a nice goodie).
If you want to use it as a navigator (think TomTom), triangulation will probably not have a high enough refreshrate. And too: GPS navigation requires constant calculations to see if you're still on the right track and recalculate the entire route if you deviate too much.. that would be too big of a strain on the battery to be useful imho.
Btw.. gps-enabling: ever heard of bluetooth-gps modules? It may just be a Software-Update away