Not trying to troll here, just want to bring attention to comments on Aperture3's metadata conventions. David Riecks Controlled Vocabulary forum is quite good reading for all things Metadata.
Article below discusses possible issues with the way Apple handles some Metadata, and whether programs outside of Aperture may not display all data fields...
Yes, there is some sort of issue. In plain english, if you fill in the IPTC metadata in Aperture, export a version of the file (e.g. jpeg, tiff, or psd) and then open the file in Photoshop CS4, not all of the metadata be read or saved if you work on the file in PS. You would have to reenter the metadata via PS' File Info window.
Apparently Bridge can read more of the metadata (which points to PS and Bridge not working the same way) and the Apple 10.6 OS can read all of the metadata (open a file exported from Aperture in Preview, hit command-I, to see the metadata.)
As David points out there is some question as to what the issue is here - if Apple's jumped the gun with a new standard that's not widely in use yet, if Photoshop hasn't kept pace, or if Apple needs to be more compliant with older versions (probably the latter given that PS is the industry standard). Regardless though the problem is there.
It could be a bug or or an oversight on Apple's part.
Best way to deal with it, if you use Aperture, is to report it to Apple
. The more who do the quicker they'll deal with it.