If I crop a JPEG photo in iPhoto and then export it as JPEG, is the exported version re-compressed a second time, or does iPhoto somehow crop without re-encoding, thus avoiding further image degradation?
Every time you save a JPEG file, you have to re-compress it. It's likely that if you save it at 100% quality, you won't be able to see any quality difference from the original. This is because the first compression already removed all parts that are invisible to the human eye. Another compression might remove something more, but I doubt it will be noticeable. So, I don't have a real answer to your question, but I'm pretty sure that you won't notice a second compression.