Sandra Bullock - who is not Jewish - has told People magazine that her adopted son Louis had a "bris", soon after he was adopted in January:
A friend of ours helped arrange for a bris at the house, because we couldn't go [to a hospital for the procedure]. The mohel [a person trained in the practice] came to us. You have never seen adults more panicked about what was about to happen to their son, but the celebration and the amount of love we felt and the pride in the little man whom we love so, so much became the greatest moment I have ever had in my life.
UPDATE: I am reminded that a few years ago, there was a trend of non-Jewish kids celebrating 'faux barmitzvahs'. Are we going to witness the rise of 'faux bris celebrations'? Somehow I doubt it.