There's also the Kodak DCS ProBack 645M, produced from 2002-2004. Some people are frightened off these because Kodak no longer supports them, but I got mine not so long ago for mid-$2k, only 12,500 shots taken, no moving parts, and it had just been serviced at Midwest Camera, so I don't see what the problem is. It's shooting beautifully and has some great features like a click-on, click-off removable IR filter that you cannot get on a PhaseOne or Leaf back (the Mamiya ZD camera/back is the only other one that can do this).
And hey, merely repairing the Firewire port on a PhaseOne back costs the same as my entire Kodak back kit, so I think it's good value.
I looked at the ZD back and all the other backs as well (I did up a massive spreadsheet), but I was not impressed that everything apart from sensor size is worse on the ZD: lower quantum efficiency, lower quality at > ISO 100, higher dark current.
Mr. Luminous Landscape himself, Michael Reichmann, liked the Kodak so much, he bought one for his Contax 645 (see reports here at LL).