Watch a video of a baby being circumcised. Trust me, it's a big deal for them. Just because they don't remember it later doesn't obviate the screaming and apparent agony they face when they're chopped. At least as an adult you can get anesthesia -- which is not available for a newborn.

Honestly, I think the practice is barbaric. What reading I did when I was faced with this question indicated that the popularity of circumcision in 20th century America had a lot to do with early-century anti-masturbation hysteria.

If there's real interest, I'll dig up my old research and post it here.