If this was based on atime instead of mtime, and was coupled with Time Machine backups, it'd actually make an interesting form of Hierarchical Storage Management. Files would "expire" from your local disk, but still be restorable.<p>On a tangent, that's really what I (and I think most people) want from HSM—not "files canonically being on slow media but being cached on faster media", but rather "files canonically being on small/fast media, and then migrating to slower media when you stop caring about them, as if a garbage-collection pass had occurred, leaving your disk with more space." Basically, HSM should do automatically what people do manually when they e.g. burn files to optical disks to clear up space.