There's absolutely nothing wrong with taking pictures whilst mid-flight. There are no specific laws to prohibit you from doing so, though the airline can set it's own rulings which you do need to adhere to.
Now if you're taking shots with the lens pointing inside the cabin that's a different matter. A number of airlines may stop you from doing this on the grounds of securing the privacy of fellow passengers. They didn't get on board to have their picture taken did they !
If a fellow passenger was bothered by you taking shots out the window I'd politely ask him to mind his own business, though you're more likely to encounter that if travelling within the US as we know how paranoid the yanks are when it comes to aeroplanes or should I say airplanes !!