When trying to move a permission between Current and Available a Javascript error occurs.
I've only tried this with Files and Folders in the document library.

Doing the exact same thing works fine in Firefox 2.0 and IE7.

Sorry I don't know how to capture the Javascript error text. IE6 won't let me select it to copy. Here's what I see:
Line: 487
Char: 1
Error: Object doesn't support this property or method
Code: 0
URL: http://localhost:8080/group/11005/1?
   p_p_id=86&p_p_action=0 ....

The actual URL was very long. I wish I could copy it but IE6 doesn't let me select anything in that error dialog for copy.