I'm thinking about Sandisk Eye-fi that have wi-fi geotagging option combined with iphone sending GPS info thru wi-fi to Eye-fi.
I have the Eye-Fi as well. There are a few issues:
1. Already mentioned is the fact that itisn't actually a GPS. It uses a database of WiFi hotspots to guess where you took the photo. If there is nomWiFi, there is no data.
2. It only adds GPS data to the files during the transfer to the computer and only then if there is an active Internet connection.
It is actually quite complex in the way it works. As long as you have the WiFi enabled on the card when you take a photo, it adds the details of nearby WiFi networks (even ones you don't actually have access to) to the file. When you use the EyeFi software to retrieve the photos from your card it queries an Internet database (SkyHook) and determines where those WiFi points were and translates this into GPS data in the EXIF. The latitude and longitude data did not exist until this point. If you do not have intent access when you make the transfer there is the option to get this information later, but only if the photo remains in the Eye-Fi software (and not on the iPad app). Once you send the photo to Aperture or Lightroom etc you can't add the GPS data to that version. This becomes a problem if you use the ad-hoc connection the card creates to transfer your photos as the receiving device then connects to that WiFi and so can't connect to an Internet WiFi. You also don't get GPS data if you transfer the photos using a card reader, it must be a wireless transfer off the card.
Don't get me wrong, I think the Eye-Fi card is great. I just see it as a clever way of transferring photos, rather than as a GPS device. The fact it does allow a sort of geo-location is great, but you need to be aware of its limitations. The GPS data added from the GP-E2 is transferred via the Eye-Fi card with no issues.