One of the problems with Final Cut Pro is that once you save the project file, it can only be opened up again on systems running the same version number of the software. So, if you were working on a machine that had FCP 7, and took your project to another machine running FCP 6, and tried to open up the file...no dice.
Luckily there is a way around that problem. XML enables us to save our projects in a format that will let us continue to edit on other machines with older versions of the software. Steve Jobs might not like this idea, but I am sure you will.
Here are the version numbers for you to use.
Use XML version 1 for going back to Final Cut Pro 4.1
Use XML version 2 for going back to Final Cut Pro 5.0
Use XML version 3 for going back to Final Cut Pro 5.1.2
Use XML version 4 for going back to Final Cut Pro 6
Use XML version 5 for moving between Final Cut Pro 7 systems