Posterous places nonremovable canonical URL tags on its posts. That tells the search engine to assign all the “link juice” to the Posterous-hosted page no matter where else the content may exist.
I do not want the Posterous version of the document to be the official URL for much of my content.
I hadn’t spotted this, but I’m glad I know now.
This could be a problem for you if you duplicate your own content on a Posterous blog, but you don’t want Posterous to be the main home for that content. It is very likely that your Posterous blog will rank higher than the other place, whether you want it to or not.
In truth, it’s probably not an issue for most users. Better to know and not care than to care and not know.
Further reading: Learn about the Canonical Link Element in 5 minutes – mattcutts.com