Over on the Email Responsibly blog, Experian CheetahMail's Ben Isaacson explains "that Experian CheetahMail believes that opt-out email appending is no longer an acceptable practice, and that marketers should no longer use this practice to acquire customer email addresses."
For those of us banging the best practices drum every day, this is fantastic news. For an email service provider like Cheetah, who has seemingly engaged and supported the practice for many years, to stand up and say yeah, it's played out, don't do it, this has to signal a major shift in the industry.
Some, but not all, email service providers have banned use of email append for some time now, and a rallying cry disaffected clients, when told not to utilize it, was often "but company X would let us do it!" The list of company Xs that would allow that sort of thing has just shrunk significantly today.
ETA 1/25/2012: Ken Magill covered this in the Magill Report this week.