I sent in my T420s and it was returned on February 29th. Informative updates from Mark_lenovo were:
"""We have it and have made a bit of progress. We pulled out the Linux drive and set aside for safe keeping and installed a Win 7 factory preload drive. Initially all was well, but after applying all Windows updates and applicable Lenovo updates, we are seeing the system CPU temp increase while idle and then the fan ramp up and run at higher speed, and then would settle back down. The engineers are still testing. """
and:
"""The engineer has ordered a replacement system board and fan and will change them out on your system so we can return it this week. We can then keep the parts. """
My T420s with new system board and fan has actually been performing better, though it is not fixed.
On battery, the fan will modulate up and down, though it can still get stuck at 4000 rpms after a while. However, it used to shoot straight to 4000 rather promptly, so it is an improvement. I do see ~3600 rpms some times.
On AC, the fan is still apt to get stuck at 4000 rpms.
Has anyone noticed that even at 4000 rpms, the air is not blowing particularly strongly?
Interestingly, last night I ran powertop and it listed the fan (at 4000 rpms) as the biggest power draw. Since the return of the laptop, I investigated some Linux powersaving options mentioned earlier in the thread. I was not scientific about it, so I don't know if that was a factor in improving the fan on battery. Mainly, I think I just installed tlp and updated to a 3.2.9 kernel with the ASPM fix. They helped extend the battery life, so I am more likely to run off battery. That made me wonder if the power consumption rate is a factor in the BIOS fan control. Prior to improving the battery life, I was more likely to run off AC which did the 4000 rpm thing.
FYI, my T420s has integrated graphics. BIOS version 1.30. It was originally manufactured in April/May 2011, but I am not sure of the replacement system board and fan dates.