My best friend is Jewish, and I have been involved in most of the most important events in his life. I was the best man at his wedding, I celebrated Hanukkah with his family, and I am the "godfather" of his son (although this last item is purely honorary, and if my friend and his now-ex-wife died, I'm sure he would be adopted by one or the other set of grandparents).
I was asked if I wanted to attend the bris, and even to participate in the ceremony, but I just couldn't bring myself to do it - I'm not especially squeamish, but I think seeing the circumcision taking place would have been too much for me to handle. I politely excused myself from the event.
I'm not sure if it is generally accepted for a non-Jewish person to take part in a bris. My friend is almost totally non-observant (not only does he cover himself in tattoos and eat whatever he wants regardless of kosher laws, he even celebrates Xmas), so I am wondering if his invitation was possible only because he doesn't care about the accepted practice among observant Jews.
Is it acceptable for a non-Jewish person to participate in a bris?