Mac OS X 10.7 includes a new facility to make Time Machine backups on the local disk. You can make use of this feature to create snapshots of the current disk status, allowing you to recover instantly from disaster.
Local Time Machine backups should be enabled by default on portable Macs, but on desktop machines you'll have to enable them manually, in Terminal:
sudo tmutil enablelocal
Once local backups are enabled, you can make a snapshot almost instantaneously by issuing this command:
sudo tmutil snapshot
This is a very useful (and prudent) thing to do immediately before embarking on some course of action that could have potentially disastrous results. If disaster does strike, you can simply enter Time Machine and restore the affected files or folders to their state at the time that you took the snapshot.
Since the backup is local, you can do this even if you're not connected to your Time Machine drive.
[crarko adds: If this is a full Time Machine backup expect it to take up a lot of disk space. I don't have enough free space to try this but a look at man tmutil confirms these options. There are many others, including setting up exclusions, so it would be well worth a person's time to look over these man pages to get a better handle on this utility.]
Mac OS X Hints