Suddenly, when shifting from AC to battery, I noticed that the problem is gone. I did install a new monitor driver as well as the new power management driver about a week ago, but that didn't make a difference. I've also installed a couple of applications, but nothing that should have any effect on the Intel graphics drivers. Also, I installed an updated driver for Thinkpad bluetooth with enhanced data rate and that turned on my bluetooth antenna by default (it was always off before). But I've done nothing to solve the problem.
Somehow it is gone now. I've tried to dis- and reconnect the power cord multiple times, even after restart, and it seems the screen works fine - it doesn't shut off. Turning on or off bluetooth radio doesn't have any effect either.
It is a mystery. But I am very happy that this annoying screen shutoff is gone.