I believe the "real time cloning" Frank refers to is actually running a CARP cluster - having a redundant box ready to take over in case of issue with the primary. To do this you need a spare box, as well as a spare IP address on LAN and WAN side for the secondary router, and then another IP for the CAP virtual IP on each side. Works very nicely - state tables are synced so sessions stay open, but first time you fail over Windows 7 / Server 2008 machines will detect a new network due to different MAC address for default gateway, and traffic may stop until the machine is told whether it is a home/work/private network.
You could take an offline image using something like Clonezilla or Acronis. This should work as *nix builds are less fussy about being moved from one box to another than Windows - even on the pfSense forums it's recommended for people having problems installing to remove the hard drive, install with the drive plugged to another machine, then bring the drive back to original machine.
However I've found pfSense installs pretty quick unless you have a massive hard drive. I prefer using the backup/restore functionality mentioned in the question as it means the file restored to backup is much much smaller.