This isn't news to many hardcore Mac users, but I have to point this out to the uninitiated.
The other day I wound up needing to mirror the boot volume of an XServe. It was one of the older G4 models with four drive cages in the front running Mac OS X Server 10.3. You can't create a software mirror on OS X and retain the data on one disk, so I would have to image the boot drive, create the mirror, and restore the image to the newly created RAID1 array. On most other operating systems, I would be restoring from tape at best, or reinstalling at worst.
I booted the XServe via FireWire from my 12" PowerBook running OS X 10.3, used the Disk Tool to create a .dmg of the boot drive on the laptop disk, then created the RAID1 array with that drive and another disk in the server, and restored the backup to the new RAID1 set. The server rebooted from the new mirrorset without any problems and all was well.
Total time: 20 minutes. On a G5 server with SATA disks it would be faster still.
That's just sexy.