"Control commercial spam" is a Google Checkout value proposition. "You can keep your email address confidential, and easily turn off unwanted emails from stores where you use Google Checkout," reads my account page.
Didn't work at Starbucks, where I couldn't even find a Google Checkout option. But check out the new Buy.com checkout page (left). Bypassing all that merchant registration tedium really will be advantageous for e-commerce players. I've seen this functionality on some, but only a very few, Yahoo Shopping merchants.
This is a concept we'll be seeing more of in the future. How will e-mail marketing programs adapt? Under CAN-SPAM, businesses can e-mail existing customers. What happens when businesses no longer have their customers' e-mail addresses? (Google is, after all, calling this "commercial spam").
Merchants are going to have to get very creative at developing incentives if they expect to build their e-mail lists in the future.