This May, I corresponded with Canon's Chuck Westfall about the 50/1.2 focus issue. His reply is below. The problem as I understand it is that the lens will be correctly focused at f/1.2 (it always focuses wide open), but when the lens stops down for exposure, the focus will shift to the rear, perhaps as the result of spherical aberration. Thus, manual focus will not solve the problem unless you manually correct by focusing slightly closer. According to reports, the problem is worst at f/2.8 at a focus distance of a few feet.
Here's Mr. Westfall's response to me, dated May 24, 2007. To my knowledge there has been no further response from Mr. Westfall or Canon about this issue. My guess is that the problem is inherent to the design of the lens, so there is no easy cure, and perhaps no cure at all.
Canon Inc. is definitely aware of the claims concerning the AF accuracy of the EF50/1.2L USM. They've been investigating for the past couple of months, and I am told that there will be some kind of public announcement forthcoming in the near future. Stay tuned, and thanks for using Canon equipment!
Director/Media & Customer Relationship
Camera Marketing Group/Canon U.S.A., Inc.