It seems like cloaking devices would have to bend a lot more than visible light- they go completely invisible to all sensors, which means they must avoid radar and all that other stuff as well. And I'm pretty sure it is a bending of the light; you always see stars behind the ship as it shimmers away, and there's no shadow right before one uncloaks. As far as I can see, if there's no way to see in, there's no way to see out.
Of course, that doesn't stop Klingons from sitting at the bridge of their U-boat -- I mean, "Bird of Prey", and launching torpedoes at the Enterprise. Maybe that periscope really does have a purpose...