Gary, yes you can rotate the near 645 digital backs on the V cameras. You do not turn the camera on it's side like you do with the 645 cameras. Two MF cameras offer rotating mechanisms so the digital backs do not have to be removed to change from Landscape to Portrait ... Mamiya RZ and Rollei (Sinar Hy6)
How long a digital back will last is not known. I once talked directly to the head of Kodak digital services and he said there was little to mechanically wear out. Things like the firewire port connection can come loose and stuff like that maybe.
Used digital backs are available from Kodak (Proback and Proback Plus); Leaf Valeo and Aptus; Phase One, Eyelike; Imacon prior to Hasselblad merger; Hasselblad CFV.
As far as image quality is concerned, just about any one of these older models will outperform the latest high meg DSLR. The Kodak Proback produces better images than the Canon 1DsMKIII ... trust me, I owned both. Nothing made by any 35mm DSLR maker can match the file quality from a CFV, let alone a 39 meg digital back.
I do not agree with Blowupster that MF digital is out of range even for the Professional. While film and camera/lens gear is much cheaper than MF digital, film, processing AND professional scanning is not.
Virtually ALL reproduction of professional photography is now digital and requires digital files be provided to the printers. Clients no longer will tolerate scanning charges for a majority of professional work. For example, a common catalog shoot with 100 images @ $50. per scan average = $5,000. plus film and processing. A used Kodak ProBack Plus can be had for $4,000. or less. It does not take many jobs to pay off a digital back.
The second considerable difference is that where the film, processing and scanning charges were passed on to the client and were outside expenses with no income captured by the photographer, most Pros now charge a digital rental and capture fee which is paid to the photographer IF they own their own equipment.
The day rate to rent a MF digital camera is at least $500. or more a day. If the Photographer owns their equipment they capture that rental fee to help pay for the MF back. 50 days of $500. rental fees pays for a $25,000. MF digital camera. That is just one job a week per year.
This is one of the reasons I've said that MF digital is geared toward the professional photographer, and less so the advanced amateur. However, if one doesn't have an obsession with buying new, the price of digital backs becomes more attainable.
A Phase One P25 back for a Hasselblad V camera can be had for $10,000. This combination will outperform a $8,000. Canon with no problem.