ClickZ's Al Diguido considers Google's Gmail service to be a burr under his saddle. It rasps his sensibilities that email marketers could have their communications displayed next to contextual search listings that could include competitors. Among the possibilities he considers is stopping delivery to customers who have Gmail addresses, although he suggests testing emails first to determine if there's an adjacency problem first.
Fine by me if you don't want to send spam to my Gmail account. In fact, I remember even suggesting something like this back when Gmail first came out. In fact, here's a quote from that post:
Here's a suggestion for 3rd parties sending e-mail to Gmail users: set your spamming, I mean e-mailing engines to not send mail to *@gmail.com and *.*@gmail.com. By doing this, you will be able to opt out of having your e-mail scanned.