After reading Michael's review, it struck me that the answer is "maybe" - it highly depends on your needs, and your pocketbook. Can you get medium format quality work out of a 1Ds - yes. Does the Pro Back 645 offer the same level of quality, with a little more flexibility on cropping, etc - yes.
The price difference is what really jumps out at me, though. For the price of a medium format camera and the back (no glass included), you can buy a 1Ds, and a nice assortment of glass and accessories (maybe even a backup 10D, if you wanted).
I'm looking forward to Michael's reviews of the 22MP backs - I suspect we'll see medium format digital really show what its capable of with more sensor resolution.
Hey, Michael, when are you going to review large format backs??