When I import photos to iPhoto, it uses file modification date instead of EXIF Original date. iPhoto's Extended photo info (⌥⌘I) shows no EXIF data. However, when I inspect the file using Preview.app, there's EXIF information with correct date.
I'm not 100% sure, but I suspect this broke when I upgraded OS X from Snow Leopard to Lion.
I was considering installing iPhoto '11, but I don't really want to pay 12€ just to fix something that used to work properly.