This feature would be really helpful for many users.
Having a setting in Preferences letting us to save a project with all the media stuff in at least 2 different locatons automatically is one of the features I'm waiting from Reaper since I always work in my studio on a desktop, or at home or while travelling on a laptop and I want to be sure that I've got everything to continue working with a project at any moment in any place.
By far I have to always use "save as" after the main saving and backup my project folders in different locations using outside utilities to have everything I need. It's easy to forget to copy something and then it's impossible to work untill you get your main computer.
And the feature would solve all of these problems very easily and elegantly.
Please, vote for this feature here -
http://forum.cockos.com/project.php?issueid=5626