As of MacOS 10.3 Apple moved away from the binary encoded nib files, and towards the xml encoded XIB files, for their interface definitions
The xib file format is actually quite easy to understand, it is the responsability of the object encodeing and decoding, it to insure that it's attributes are read correctly.
Here is a commented example of a XIB file. For easy reading open it in a syntax highliting editor using XML highlighting.
Todo figure out how "</bytes>" is escaped when encoding a byte stream.