Use of iPhone GPS Overseas

Technically, yes - in that the GPS will still triangulate your position. Practically, not if you're using the Maps app or any that download data to provide context for the position data. So your photos will be geo-tagged, but you won't be able to see where you are on the map.

When I'm overseas I make a bee-line for the nearest vendor of pre-paid SIM cards. IIRC I paid £10 (or £15?) in the UK for a pre-paid SIM card which gave me enough minutes and data for a two-week UK visit a few months ago. It's a bit of a pain managing incoming calls to your main number (I just used voicemail), but saves a heap and maintains connectivity goodness.

Hope this helps.

If you use a GPS application with map caching abilities like MotionX GPS, you can download e.g. Openstreetmap maps for the region of interest and use the app for map viewing and track generation.