Use a Constant Theme
When all of your content is able to fit within the same constant theme this makes it easier for search engines to recognize the difference between the theme and the content itself, and in the long run makes it easier for search engines to index your content. If possible, keep your theme consent throughout the entire website.
via shayhowe.com
Just saw this on a list of ways to get better SEO -- turns out using one common theme for all content makes it easier for search engines to determine what's content and what's formatting.
Posterous has only one theme (for now), so we certainly take advantage of that SEO quirk.
Read more about how SEO is affected by domains and URLs here.