My 4S(iOS5.0.1 still) doesnt do this BUT my iPad(1st gen) on the other hand does !! i'm STILL on iOS4.3 with it as i havent had time to upgrade it YET, but it certainly does drive me crazy when it does peek-a-boo on me. Not too often, but if i had to estimate, maybe once a week ???
Idk what causes it on my iPad either, but perhaps if you reboot your iphone it may help. Worth a shot unless you've already tried & it didn't help. Only other suggestion i could think of is maybe do a restore on your phone......(i know, prob not something you really want to do either lol)