If you want to do it completely for free there are various software systems that allow you to take the GPS tracks logged on your phone and automatically add them to the EXIF data on your photographs (as long as your clocks on both are set the roughly the same!)
Ok its a bit more geeky and involved than putting a GPS unit on your camera, but its a valid alternative. However, as I only think its really only of marginal interest for a handful of my images, I tag them manually with the interactive map on Flickr when they are uploaded, doesn't actually take that long as exact accuracy isn't important... and sometimes you don't want the exact location known anyway for whatever reason....
