OS X Lion has tweaked Apple's Time Machine backup app a bit, but the changes are pretty significant. Now, if you're away from your external backup drive or Time Capsule for a few days, Time Machine actually caches a local "backup" of documents and system changes on your boot drive, waiting for an opportunity to transfer the backups to your full-time safety net.
I first encountered this feature when I was using an early development version of Lion and accidentally clicked on the Time Machi
