A few pages back there's this topic :
this page which also mentions it. I haven't yet seen any certain information about it, but the OP seems to have a pretty good idea of when it's a problem for him/her.
Maybe I've just been lucky, but it hasn't been a problem for me yet. I'm fairly sure that if you throw a summoned monster between yourself and the enemy (as I tend to do), the bulk of that creature will provide cover for your characters. So, for now, perhaps you might consider using summons as LOS shields.