Merging project/solution files is a well-known disaster among developers/SCM admins performing merges in their source control.
Take, for example, a common scenario:
Project: Merge is my tool for comparing and merging XML files. I originally wrote it because I was experiencing exactly this problem with Visual Studio project files.
It correctly detects re-ordered elements and/or attributes within the XML file and will correctly resolve nearly all 'conflicts' automatically.