I had the exact same issue after upgrading from 2.1.X to 2.6 tonight and found this post. You were correct – commenting out that define statement seems to remove the stray characters in the posts.
Kind of strange. Unfortunately I don’t have time to troubleshoot as to why this was happening because this was a blog that a customer was paying me to manage – not the blog their domain/website, the blog just happens to reside on the site.
I hope the good people at WordPress take note that this is definitely happening and not to just 1 person.