[SEE UPDATE due to changes in a Snow Leopard patch]
I've finally completed a whole RAID 1 backup cycle with Snow Leopard and I can reliably report on how it works.
The process, when performed reliably, is essentially unchanged from earlier versions of Mac OS X. [Details added 3/4/11].
Specifically, you must never attach an old software RAID 1 drive to the working RAID 1 set. If the set was missing a drive ("degraded") before you attach the drive, it will treat the new drive as part of the set. THIS IS BAD.
You must always erase a drive's partition header completely before adding it back in to a RAID set. Otherwise it's misidentified as being an up-to-date part of the RAID 1 set even though it may not have been updated in months.
I had thought that changes made to RAID handling in Snow Leopard might have fixed this problem. Nope.