I just came across something interesting, at least it is to me 
because I haven't heard of this before.  It appears that MacOS X adds 
resources to files.

I was looking at the resource forks of a few files I've created and 
noticed a resource type I've never seen before.  I found it after 
changing the application binding, the 'Open With Application' menu in 
the Get Info dialog.  The resource type is 'usro', ID 0 and 1028 
bytes in length.  I can't find any mention of it on Apple's website.

It turns out that it's holding the absolute UNIX style path of the 
application that opens that document.

Since it's ID is 0, IIRC Apple reserves all IDs below 128, it 
shouldn't be a problem.  I did find that if the Finder finds a 'usro' 
type with the same ID but without the correct information it will 
overwrite the original.  That is unless its an empty resource.

I deleted the resource's data and tried changing the application 
binding.  In trying it the Finder gave me a 'The operation could not 
be completed' (error -39) the first time and a second try caused the 
Finder to crash.  That error is an EOF error so that makes sense.

Worse is if the resource contains data.  I filled the resource with 
some random hex data, more than the Finder did.  The Finder overwrote 
_some_ of the resource with it's own data but didn't zero out the 
unused portion.

In addition, if the application isn't the default application, the 
Finder adds a 'icns' type resource (ID -16455) of the other 
application.

I just thought people might want to know.
-- 
Heather Donahue