I had an opportunity to discuss the mirror lock up issue last week with Claus Molgaard, Vice President of Research and Development at Phase One. He assured me that they are working on a fix, which he said should happen before the end of the year. His explanation of why they think it happens is interesting. He claims they have determined that it is timing problem, related to the sleep mode of the back. If (when in mirror up mode) the shutter is released just at the moment the back is going to sleep, it will lock up. He thinks it may be that the shooting style of some users makes the problem more or less obvious. In other words, if you happen to press the shutter release lightly before actually making an exposure, you would be preventing the back from going to sleep before actually releasing the shutter, thus avoiding the bad timing scenario that triggers the lock up. If you shoot without first lightly depressing the shutter release, you run the risk of coincidentally pressing the shutter release at precisely the moment the back is going to sleep, which is what causes the problem. I haven't tried out this theory yet, but I hope he is correct.
He also said battery amperage has nothing to do with the problem, although using depleted batteries may exacerbate the situation. Also, he mentioned that they will be coming out with a rechargeable battery pack as an option for use instead of the AA batteries.
I'm curious if those who have minimal lock up problems tend to routinely wake up the back with a light depression before firing the shutter? I know I usually don't, so I'm hopeful that adopting this habit will help until they fully resolve the issue.