You modify a NIB file with Interface Builder and save it. Interface Builder actually deletes the previous NIB and recreates a brand new one. So any .svn folders inside are lost. To resolve this problem, open a Terminal and enter:
defaults write com.apple.InterfaceBuilder VersionControlDirectory "(CVS, .svn)"
That will make Interface Builder copy the .svn folders to the new NIB file.
Some files in .svn folders are read-only. Because of that, Interface Builder believes the whole NIB is read-only. To fix that, add writing right to the NIB's files. At your Xcode project's root:
chmod -Rv u+w *.lproj/*.nib/.svn
Sadly, you will have to do that at every update/commit. You can add this alias to your .bashrc:
alias nib="chmod -Rv u+w *.lproj/*.nib/.svn"
Largely inspired from: Subversion and Interface Builder