Warning
Snow Leopard doesn't seem to like mirrored RAID sets created in Leopard.
Drives used: WD MyBook Essential 1TB, Fantom 1TB. Might be just a drive issue (but both??) and (more likely) an SL issue.
So back up your data on your Leopard-created RAID sets and then re-create them in SL.
IF SL hangs, freezes, or otherwise requires you to reboot because it does not recognize your Leopard-created RAID set, DO NOT delete the set in SL. It will destroy all partition and directory info on the drives. Your data might still be there but you might need Data Rescue II or a similar utility to recover it.
Install or re-install Leopard, back up your RAID set, and proceed.
Most people really don't use a RAID configuration in OS X, especially when it comes to notebooks and home-use iMacs (I assume), but for the few that do, it looks like (at least in my case) SL did not like RAID sets created in Leopard.
It's either that, or I failed to note that you should back up RAID sets before doing a clean install of a new OS, and then re-create the set in the new OS. I'm a RAID noob anyway, but I assumed OS X would recognize RAID drives created in earlier versions.
Got all my data back, though.