Mac OS X Lion Time Machine capable of 'Local Snapshots' without external hard drive
Dubbed 'Local Snapshots,' it sounds like it will be very similar to Versions, another new feature in OS X Lion that keeps track of changes to documents, and lets you revert to older versions. Local Snapshots, on the other hand, will take a snapshot of every modified file on your hard drive every hour. It sounds like it could drain an awful lot of system resources (CPU time and hard drive space), but we'll have to wait and see.
If anything, it just sounds like this is a convenience measure more than anything else: we should make daily backups, but plugging in an external hard drive every night is a pain in the ass. With Local Snapshots, you can go a few days without plugging in -- and when you finally do, Time Machine will copy all of your local snapshots to the external drive.