Editing iPhoto-specific metadata Perpetua:2011-01-16 ashley$ mdls IMAG0107.jpg
  
  
    ...
  
  
  kMDItemComment                 = "Cat with is tongue out."


So, here's my question.  xattr can be used to modify some attributes.  For example:


Perpetua:2011-01-16 ashley$ xattr -l IMAG0107.jpg 
com.apple.metadata:kMDItemFinderComment:
00000000  62 70 6C 69 73 74 30 30 50 08 00 00 00 00 00 00  |bplist00P.......|
00000010  01 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00  |................|
00000020  00 00 00 00 00 00 00 00 00 09                    |..........|
0000002a
Perpetua:2011-01-16 ashley$ 



But this isn't the attribute I want to edit.  I imagine this is because kMDItemComment is an iPhoto-specific piece of metadata.  My question would be, how do I go about editing it?
com.apple.metadata:kMDItemFinderComment is in binary property list format. Using xattr -p -l -x | tail +2 gives you just the hex dump of the attribute. You can then pipe that into xxd -r to turn that back into a binary file.

From there you can open that file in Apple's Property List Editor, assuming you have the Developer Tools installed. You can then edit the property visually.

You should then be able to reapply the edited value to the file using xxd -p -x and xattr -w -x.

This is all much more messy than one really wants for a single property that's just a string.