This question is still getting a lot of upvotes. So look at Update3. once you have upvoted, commented, answered, please please please file a radar
Try updating the XIB's "Document Versioning: Development" setting:
Xcode: File Inspector: Interface Builder Document: Document Versioning: Development
to at least "Interface Builder 3.1".
Version 4.6.3 of Xcode resolves this issue. However, as the comments state, you all need to be running that same version. If four developers are on 4.6.3, and one is on a previous version, then that developer will experience the same issue. However, if they do not commit that change then it will not affect the others.
The two Xib keys affected are:
<string key="IBDocument.AppKitVersion">1187.39</string>
<string key="IBDocument.SystemVersion">12E55</string>
So, bottom line, get everyone on the team to update to Xcode 4.6.3.