I'm not saying this has happened, but from personal experience...make sure that you know and trust the people you are sending your essays out to. I would very hesitantly send my PS to someone on a message board whom I did not know. There's no way to tell if they are dishonest and may take it for their own.
In my honors philosophy class this past semester, we had a final thesis. We spent one class meeting switching around papers so that we could offer constructive criticism. One girl missed the class and IMed both me and my roommate asking us for our papers so that she could see if she was on the right track. I didn't email the paper but just gave her an idea of what I was doing. My roommate emailed her entire paper. Imagine her surprise when a week later, they were both brought before the academic disciplinary board. Apparantly the girl submitted my roomate's paper. The teacher noticed immediately that there were two submissions of one paper.
It'll be a lot harder to admissions counselors to identify this, but I wouldn't take the chance. You never know.
I'm not directing this at anyone, but I've seen a lot of activity regarding PS swapping the past few days. Just be careful.