The problem with cell phone cameras is that people can take them anywhere. The common example is a locker room, but I read today about a stage manager who used his iPhone to spy on an actress in her dressing room. Hopefully I don't need to point out that being a voyeur is not only impolite but also, generally, illegal. However, less egregious examples are worthy of some etiquette-oriented reflection.
Having a camera constantly on the ready prevents a lot of "darn, I wish I had brought a camera moments." That combined with the essentially free nature of digital photographs enables a whole lot of pictures to be taken that might never have been captured in the past. Additionally, the Internet allows for these images to be shared widely. This is great for showing the grandparents a baby's first steps, but it also results in the journalistic recording of far less noble events.
I know they are funny, but please don't publish embarrassing photos of your friends without their consent. And if you publish a photo that a friend wants to be "untagged" from or wants to have totally deleted, please respect those wishes. (True, nothing is ever truly deleted from the Internet. Cached archives get generated too quickly, and you never know who may have done a right-click>save already. Still, you can minimize the damage by removing something quickly.) Realize that your definition of embarrassing may vary from that of your friend, or they may want the picture removed for another reason. Perhaps they are avoiding a stalker. Perhaps they are trying to establish a false alibi. Perhaps it just makes them look fat. Regardless, the polite thing to do is to give them some control over images of them put on the web.
Additionally I add this even though few will heed it: Please don't post embarrassing pictures of strangers. I know we are a voyeuristic society, but most of you won't think "People of Walmart" is nearly as funny when you find yourself featured on it. If a photo goes viral it can make its way back to the subject who may not be as amused. If someone is amused by an embarrassing photo that person is probably a reality-show-seeking exhibitionist, who should not be encouraged. The classy thing is not to mock strangers in a public forum. (In private forums, however, this is called "wit" or "social satire" and will usually improve your cocktail party banter.)