in PR 1.1, when the "ear" sensor was covered (as in I placed my N9 into the leather case, so can't see the clock anyway anymore), my N9 would the smart thing - it would turn the stand by clock on the black screen OFF

in P.R 1.2 it does switch if off at all

Maybe the consumption of the clock is minimal, but it was the principle I enjoyed owning a SMART phone

when i cover my n9 the clock is gone. Maybe some program blocked it?

Just tested, standy clock should turn off when censor is covered, mine still does on 1.2.

On a side note, I have standby clock turned off, I prefer the deep black look of the lock screen.

Same answer as P4d0uch and shanic89 - standby clock turns off when I cover the sensor (with PR 1.2 of course).

I have found that some leather cases don't effect the sensor in the right way. The sensor just doesn't work well with leather. I had this problem in pr1.1 and pr1.2. So I just stopped using the leather case.

What you can do to test the sensor is covering the top part of the phone with you hand or some paper. If this doesn’t work restart the phone and check again.

Just tested mine and same as everyone else(except OP), goes off fine when covered, and that includes sliding it into the official Nokia pouch.

many thanks for all who answered - I have done some more testing and it works indeed however with some glitches - 1 out of 5 trials it does not work

try turning on and off in settings...

I don't think it has anything to do with leather. If it is covered then it does not matter if it is from leather or just yellow stick paper (tried it).
If it was about leather then your skin would not be good to cover it to.

It's for power saving methods. I don't really have any problems, unless you have this case that covers the proximity sensor and wants to see the clock, then you have to live with it or a different case.
It's implemented in Sleeping Screen (Symbian)