Here's another data point that may be useful. I have some photos taken with a GPS enabled Ricoh Caplio Pro G3 camera. The camera wrote the GPS EXIF data into the file so it has more data than I would have encoded manually (such as speed and which satellites were locked in) and doesn't have human errors from me.

I have attached a test photo in case anyone wants it for experiments. The only processing on the file was to reduce the size in Photoshop so it would be small enough for the upload limits here.

I loaded that picture into iPhoto version 4 or 5 back when I was running a PowerPC Mac. That iPhoto library has now been migrated to an Intel Mac. The lat/lon for it displays correctly in iPhoto on the Intel Mac as 42.00 22.00' 13.68"N, 88.00 59.00' 40.57" W. However, none of my photos seem to show anything for altitude other than 1.00 m (or -1.00 m if there is no altitude recorded.)

If I take that same photo file and load it into iPhoto again on the Intel Mac, it changes the lat/lon displayed to 42.00 22.00' 0.88"N, 88.00 59.00' 2.17" W, which has incorrect seconds.

iPhoto doesn't seem to modify the file. I can open the imported file in another application and the EXIF data seems to be fine. So iPhoto may be storing the GPS info somewhere on import.

I tried all of this with an unmodified iPhoto.