In an amusing story this past week, a New York City subway passenger was surprised and disgusted when she found a creepy guy flashing her and masturbating on a lonely subway car last week.

Thinking quickly, she snapped a cellphone picture of the guy, which caused him to panic and flee. Turning the photo over to the authorities, and publishing it on Craigslist.org and in a Flickr photo album, the story — and the picture — were soon on the front page of the New York Daily News.

Not surprisingly, the flasher was recognized by one of his previous victims and he was subsequently identified and arrested by police.

These stories are becoming more frequent, and as one who is often annoyed by the cellphone antics of rude and unthinking people, it is interesting to see cellphones being used for more positive social change. But this comes with some privacy concerns too.

For example, in a Washington Post story a few weeks ago, Subway Fracas Escalates Into Test Of the Internet’s Power to Shame, a woman whose dog relieved itself on a subway train in South Korea, was asked by fellow subway passengers to clean up the mess. She refused.

When she became belligerent, one of her fellow passengers snapped some camera-phone pictures of her and published them on the web. Others identified her, posted personal information about her, and she became the target of anger and ridicule. The woman eventually had to quit her university due to the amount of harassment she was getting.

Certainly this kind of social pressure can be taken too far, and I’m not sure someone should be hounded out of school for failure to pick up dog poop. But perhaps this will be a lesson to some people that common courtesy sometimes isn’t the highest price you can pay.