How to add GPS data to photos?
Try HoudahGeo. Full disclosure: I am the developer of HoudahGeo

HoudahGeo offers several ways for geotagging photos. My favorite is automatic geocoding using a GPS track log. Such a track log can come from an iPhone app or a GPS track logger. It has your exact position recorded every second or so. HoudahGeo matches the times on your photos to the timestamps in the track log.

Using a track log has the added benefit of you being able to retrace how you got from one point to the next. With HoudahGeo you can create a KML file that lets you visualize your travels in Google Earth with photos pinned at the locations where these were taken.

If you use both your iPhone and your digital camera to take photos at the same locations, HoudahGeo's "Geocode from reference photos" can copy GPS information form the iPhone photos to other photos taken at about the same time.