@Michael: Not sure I understand your concerns--the Mailchimp blog post agrees with Marketo's statement in that unique open tracking won't be affected, and in fact would be improved by this change. Repeat opens might be affected, so that if someone opened the email 3 times it only appears as 1 time, but Cheryl pointed out that Marketo doesn't actually include multiple opens anyway in its email analytics, so we're not losing anything there (unlike MailChimp users).
The key distinction I believe is *when* the email image is being cached, before or after email open. If it's being cached *before* the email open, then your concern is valid and open numbers would look very off. But our team thinks it seems like it's being cached *after* the email open, which means that unique opens are still trackable, and ESPs like Marketo and MailChimp can take advantage of that, which is how unique open tracking would be unaffected.