Think a lot of users use so useful tweak with name "i'm busy sms".
Bug I found - that this tweak is responsable for bug o n iphone 4s ios 5.1.1 flash blinking.
When you have this tweak active, on incoming call you always have flash blinking, which is nerwes raking and no way to turn it off. Only removing this tweak can help you.
Hope I help for some users to solve this strange bug.
P.S: can somebody reccomend simmilar app/tweak?