I have the filter-brightness-simple now working, at least most of the time. Every now and then I get the black screen effect when double-tapping the lock screen (once in couple of weeks or so). If you want to see the effect, do echo 0 > /sys/devices/omapdss/display0/backlight/display0/brightness To get screen back "echo 7 >...". When my screen suddenly turns black, something has written 0 there. Don't know what triggers that.
^ If it's reproducible..... Or you have a rough idea under what circumstances it's likely to appear, make sure you report it. http://harmattan-bugs.nokia.com/ Check that it hasn't already been reported first of course. If you get enough people watching the bug & voting, & Nokia devs can identify root cause/s thanks to good feedback. Then it's likely to be rectified for 1.2, 1.3, or 1.4 (if that happens).