Interface Builder not in sync with Xcode: can’t see images classes and project status is yellow or gray

Ever since upgrading Apple’s iPhone SDK to 3.2 beta and 3.2 final now, I started experiencing this issue: Interface Builder (IB for friends) didn’t recognize, show or list the images in “nib” files associated to an Xcode project. Other symptom of the problem is Interface Builder not being kept in sync with an Xcode project…