Prune (
http://www.activityworkshop.net/software/prune/index.html) is not really a workflow, but it is a nice Java application that works well on Mac and has quite a few controls to attach photos to GPS tracks (it uses the GPX format for the tracks, which is available from many GPS devices). If you want to save the GPS co-ordinates back to the photos again, you need to use EXIF Tool (
http://www.sno.phy.queensu.ca/~phil/exiftool/) which has a Mac DMG file to install it.
This will not work directly with iPhoto, but it will work directly on the photos themselves, and all the software is free.
As an aside, Prune is great for getting rid of all those "unwanted" sections on a GPX file, such as when the GPS device was searching for its position, or you forgot to turn it off and it kept recording, or to split tracks into separate pieces or join separate tracks into one. I started using it for this reason, then realised how great it was for photo tagging.