How much code do you need? Seriously, what's the point of asking for examples - is the problem that hard to understand? But if you really like to know, I have a pet project under development that has a File class. Unfortunately during one exploratory test it clashed with PEAR's File class. So which class name should I have changed at "build" time? The one who is used less in my current system (PEAR::File), or the one that's probably used more by other people (PEAR::File) left untouched? Should both be changed?
Your motivation was that it is "it's immensely helpful to have such a mechanism at hand if you need to integrate class architecture from different, non-related projects." So the focus in on just those cases where there are libraries that one would want to integrate that actually have name clashes your current code. Do you have examples?