These type of cases are starting to pop up here and there. There are several out there that are eSATA which have two drives internally. Some even have built in mirroring or striping so you set the jumper on the back and let the hardware mirror (for half of the space) or stripe (for twice the space but with increased performance).
The ideal setup for me would be to have an eSATA card in the computer (with 2 eSATA ports) and then hook it to two of these in striped mode (each drive is setup striping) and then mirror across the two drives for backup. The only thing I'm not sure about is if there's a single drive failure in the enclosure, if it has any type of notification. I think there's an actual RAID level value for this setup (maybe 5?) but you could have either enclosure totally fail or one drive in either fail without data loss. The data loss scenario would be loss of corresponding drives in both cases (so drive 1 in both would fail).
The server setup that I'm contemplating building will have 4 internal SATA drives, but this would be an awesome upgrade option to present to users.