After editing a JPEG file in iPhoto 08, the EXIF data is not found by EXIF.py. The data is still there, but the EXIF header is shifted further down the header package and EXIF.py only seems to look at a fixed location in the file. (In the specific case of iPhoto IPTC data is shifted before regular EXIF data and EXIF.py thus fails to "find" the EXIF.) I believe this also causes iPhoto-edited...
2009-03-12 14:11:55 UTC in EXIF.py