I find it a bit rude to take photographs of people without their knowledge, such as shooting from the hip or trying to hide the fact that you have a camera. If you feel uncomfortable in what you're doing, you should go and ask people if you can take their photograph (you'll be surprised), or just simply not hide the fact you're taking a photograph. There's no law against taking somebodies photograph in the street, you don't need their permission, just don't be secretive about it, it's a bit 'a man wearing a trench coat in the park' for my liking, haha.