This is the corner right by my apartment. It makes me laugh every time I walk by...
I can see why people would think that you had to opt-in. The button that says "Sweet! Check out Buzz" must clearly sign you up while the link that says "Nah, go to my inbox" is obviously the option to not sign up. That's just false though. Both of those links took you to the exact same place. I don't remember the exact URL, but I believe both options were https://mail.google.com/mail/?ui=2&shva=1#buzz or something similar.