I successfully used a mirrored RAID on an older G4 Tiger system with two internal drives, but according to the folks at SoftRAID, Apple's software RAID is basically intended for XServes and isn't even tested on external drives.
None of Disk Utility's help files give you any warning about this. In fact, in Snow Leopard, they don't accurately reflect the operation of the program: I saw a Demote button on the RAID setup panel that wasn't even mentioned in Help, and some of the RAID procedures described in Help simply didn't work.
I gave up on the RAID idea and instead have scheduled SuperDuper! to periodically make a Smart Update image copy of my boot drive to the external drive. Not quite as secure, but a lot less hassle! If you don't have an XServe and you really, really want a RAID, it appears that SoftRAID may be the best solution. It isn't cheap, though ($129 download).
[robg adds: If you've used Apple's RAID tool successfully on a recent Mac OS release, feel free to share your experiences. I've never tested it, so I can't confirm the above experiences.]

