This one really annoys me. I've seen friends with the colour in their status updates and I didn't know what it was until someone explained it in their status. The thing is now, several people have it explained in their status. It just says, "List the colour of your bra for breast cancer awareness." So, at least they're telling people.
When I finally got the email from someone asking me to participate, I responded with, "I'm sorry, but I don't understand how keeping a secret raises awareness of anything." I hope that wasn't rude, but it's how I feel.