Netgear "soursa" is a dual raid drive w Ethernet port. It supports time machine and is cheap at (from memory) $169 at Best Buy. It's really a NAS with a simplified interface. Wasn't quite what I was looking for, so no hands on experience. Seems they made some interface trade offs regarding music streaming (browser). But... you said RAID backups were you key need.
My Book Live II also does two drive RAID fairly inexpensively - Frys at $199