OpenSolaris + Open Storage + ZFS = Open Source storage. Not that you can’t just pay Sun for an appliance if that’s what you’re after. They have the OpenStorage 7000, but really isn’t it fun to build up your own? If that’s what you’re after Sun has the Open Storage Community page.
Using Open Storage you can go far beyond using Time Machine with an Open Solaris NAS. You can build your own truly enterprise class storage system and connect to it from pretty much any operating system. However, it’s not an appliance. Support isn’t necessarily going to be end=to-end if you BTO your own gear, rolling your own SAN. But, it is intriguing and especially if you’re looking at iSCSI storage I’d strongly recommend checkin’ it out…