I (now) know this feature has been in Xcode for years but I just discovered it. Snapshots in Xcode 4 provide fast and reliable mechanism to make safe copies of your projects.
To create a snapshot simply choose File > Create Snapshot… choose a name and optional description and you are ready to make major changes to your project! To restore to a snapshot go to the Organizer window (Window > Organizer) and browse for the desired snapshot on the Projects tab. Click Restore Snapshot at the bottom and your project is exactly the same state as it was when you created the snapshot!