Jemima's post about getting flashed in SF reminded me that I heard about this web site on NPR last week. If you've never been exposed (no pun intended) to this level of creepiness, it's hard to explain the feeling of being violated. I was flashed the first time at age 7 on a trip to NYC with my mother and grandmother. Though I didn't know what I was looking at, it was exactly like you'd imagine - the stereotypical creep in a trench coat with nothing on underneath. If he hadn't fled the scene, I think my mother and grandmother would have cut his wee willy off with nail scissors. The last time was when I was in my 20s and I had a regular stalker/flasher who used to stand in my front yard in front of my living room window with his pants down. The police actually caught him the 3rd or 4th time (I called 911 each time, but he always ran away before they arrived) and it turned out he was a neighbor that lived one street away. After I found out he'd been going through my trash and my mail, I moved.
I don't think it happens as much in these parts as in larger cities, but I think every city should have a Holla Back site. Maybe if these pervs knew that their wives, mothers, or children might see a photo of them wanking off in the subway, they'd keep it in their pants.
Update: California has a Holla Back site. So does Boston. And Texas. And Arkansas. And there was one in Seattle, but they've combined resources to form Holla Back Pacific Northwest.