Is there a diff tool to see the differences in the XIB? Can it display which graphics are in one xib and not in others?
There is no tool to do this visually. Currently textual comparison is possible (diff, FileMerge, etc.), which is still a big advantage over the older nib format.
XIB files are regular XML files. You can split them using your favorite comparison tool.
You can filter both through nib2objc (http://github.com/akosma/nib2objc) or compare the results. Then it will show you the differences in the code.