I used mondorescue with SME 6, and even saw it work on SME 7.0 using the procedures in the thread posted by psoren.
My vague memory is that mondo seems to backup just fine, but doesn't restore on all hardware configurations supported by SME. If the restore works on your hardware, great; but be sure to test it!
You can very easily do a backup to usb disk from the admin console (connect as root, run su admin and pick 'backup to usb' from the menu) that can be easily restored from the SME install CD.
If you have a spare computer you can use Affa to create a "warm spare" for any of your SME servers.
You can also use SME's native software RAID to do backups - forcefully blank a hard drive and add it to your server - SME will "rebuild" the mirror onto the new drive, after which you can take it out and have a bootable spare hard drive.