We backup our raw tapeless media (P2 and SXS) onto a shared Fibre Channel RAID from Active Storage, then back that up to Drobos. Cache-A is a great way to go if you're willing to spend the money, but a DroboPro with 16TB can be got for under $2,500. It's hard to beat that with something that gives e-mail notifications and allows drive swapping with drive failure protection.
http://www.drobo.com/products/drobopro/index.php
Just don't try to edit from the things -- they're slow. And, be aware that they're "special," meaning that if it blows up on you, the only place that can really help you recover the data is Drobo.
Good luck!
