I believe in an older version of Xcode, maybe 3.0 or even before, Interface Builder could read .nib files. These are basically compiled, like source code, and there is no way to decompress them or to see the contents, much like .png images are crushed.
Update
Found a video on Youtube, http://www.youtube.com/watch?v=dInaIlzVsn8, but have not tried it. Claims to be able to open compiled .nib files.
Update 2
Here is another possible way to open a compiled .nib. This process requires you to retrieve a .nib from Hulu Desktop application, which is not compiled, and replace it's contents.
http://www.macstories.net/tutorials/how-to-edit-nib-files-in-snow-leopard/