All,
Update - I'm editing this post to be the official solution, since it is the point in the discussion where we declared this to be a hardware defect that could occur in a percentage of the early vintage T400s systems. Discussion continued after this post as we updated stocks and cascaded communication about this issue through all service channels.
For the past several months, with the help and valuable input of everyone on this thread, we have been working to find the underlying common cause of these random power off events experienced by a subset of our T400s customer base.
We have determined that an internal noise issue with the voltage regulation component is the common element. This is an electrical noise issue which manifests in a fairly small portion of the use base and does not appear to develop over time.
As discussed at length here, some combinations of BIOS level, system settings, and driver versions can affect the frequency in which these random shut down events can occur, but these factors are not the cause themselves.
Component changes were made during the late November / early December builds and systems manufactured after this period should not be affected. Updated service parts will be in stock shortly, are now available and if you are experiencing these random shutdown issues, we recommend that you seek service to have the system board replaced. Uplevel service parts will bear 60Y and 63Y part numbers.
I would also encourage everyone with an existing T400s to update to the new version 1.12 bios regardless of whether or not they have experienced a shutdown issue in the past.
A sincere thanks to UberSteve and others who have helped to organize the information and keep the discussion on track as many combinations were tried out.
Best regards,
Mark