What does the 'property_type' key do in hook_field_info()? If you look at the hook_field_info() from Physical Fields the returned array has two keys that aren't mentioned in the documentation: property_type and property_callbacks. Can anyone explain what these are for? (Bonus question, are there any other missing keys in the documentation?!)
They're not part of Drupal core, but are extra keys provided/used by the Entity Module. The comments in entity.api.php clear it up quite nicely:

For providing entity property info for fields each field type may specify a property type to map to in its field info using the key 'property_type'. With that info in place useful defaults are generated, which is already suiting for a lot of field types.
However it's possible to specify further callbacks, that may alter the generated property info. To do so use the key 'property_callbacks' and set it to an array of function names. Apart from that any property info provided for a field instance using the key 'property info' is added in too.

I'm afraid I don't know of any other undocumented keys, but a comment in the same file just above the previous ones notes:

This is a placeholder for describing further keys for hook_field_info(), which are introduced by the entity API.

I would just keep your eye on that file when future versions of the Entity module come out to see if any new ones have been added; property_type and property_callbacks are the only ones mentioned at the time of writing.