People are confusing 6X7 format cameras and 645 cameras.
The Pentax 67 has a focal plane shutter and a large mirror. The mirror needs to be locked up for exposures longer than 1/125sec. But, the focal plane shutter is also so large that it presents a vibration issue even when the camera is tripod mounted, if long lenses are used. My Pentax 67 review is
here.The Pentax 645, for example, has a reputation as not needing mirror lock up at all, because of a superior mechanism. Until the latest model, the NII, it wasn't even offered. When the NII came out, with MLU included, I tested it and found that Pentax's claim was accurate,
it wasn't needed.The thing to be kept in mind is that shutter vibration is only a problem with the Pentax 67 becuase it uniquely is the only camera of that size with a focal plane shutter. Other 67 camera, like the Mamiya RZ series have leaf shutters, which are essentially vibrationless. So with it MLU is needed, but not because of shutter issues.
So, that brings us back to 645 sized camera, like the forthcoming ZD, which is a 645 with a focal plane shutter and MLU. If it is anything like its Mamiya 645 bretheren, or the Hassselblad H1 or the Contax 645, MLU is a good idea for long lenses and long exposures , while shutter vibration will be a non-issue.
Michael
Ps: I am hearing mumblings that the ZD body and Mamiya ZD backs may be delayed until summer '05. Nothing defimate yet, but this is what some dealers are saying.