I didn't see the letter linked to or quoted on the site. I doubt it was like JD, but it might have similarly tried to explain the trademark issue.

I've added it now. http://istumblrdown.com/letter

"The Tumblr name and mark is fanciful" -- but, we use C&D letter templates we downloaded off of the internet. They did need to send some kind of request (or risk losing their trademark), but could have done much better.