I believe in an older version of Xcode, perhaps 3.0 or even earlier, Interface Builder could read .nib files. They are mostly compiled as source code, and there is no way to unzip them or see the contents, just as .png images are crushed.
Update
The video found on Youtube, http://www.youtube.com/watch?v=dInaIlzVsn8 , but did not try. Claims to be able to open compiled .nib files.
Update 2
Here is another possible way to open a compiled .nib. This process requires that you extract .nib from the Hulu Desktop application, which has not been compiled or replaced.
http://www.macstories.net/tutorials/how-to-edit-nib-files-in-snow-leopard/
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