It looks like Google is only displaying this header for trusted companies, such as EA, since anyone else might just take your request as a cue that you want more spam.

"This only works for some senders right now. We're actively encouraging senders to support auto-unsubscribe — we think 100% should."

In the same post, Google is also letting senders of such email know how to take advantage of this new feature.

"… including a standard "List-Unsubscribe" header in your email with a "mailto" URL and, of course, honoring requests from users wishing to unsubscribe."

Gmail has always had a pretty decent spam filter – in fact, that's why I jumped on board with them as my primary email provider back in the early beta days. This is just one more reason to stick with them. I still wonder why they're only the third largest email provider. Perhaps its the same reason why Internet Explorer is still the most commonly used browser… I am change, fear me!