NewsForge has a writeup on the new MapFS filesystem, and some of its uses. One use looks very similar to UnionFS or Copy-on-Write, which already allow users to modify any files on their LiveCDs.
One possibility Dennis and Stahl mentioned is a set-up that can run off a live CD. According to Stahl, they would like to see users have the ability to change things that run off a CD, and MapFS would make it easier to facilitate such a use. He said that by populating the view so that it refers to all the files on the CD, and setting up a temp directory to be used for the copy and write target, users would be able to modify any of the files on the file system.