topherchris

From the official Tumblr blog:

Permalink URLs now include a summary of the post content.
A new {PostSummary} tag is available for posts that don’t have titles.

To be clear, I never had a problem with the SEO of my Tumblr-based sites. Maybe I’m just old-school, but I tend to believe that people will find your content if they really want to find it. Nevertheless, I’ll certainly tweak my theme to use the new PostSummary in a meta-description tag.