this story about Posterous having some issues with its SSL certificate. Those issues have since been resolved, but the fact remains that Posterous was acquired by Twitter last winter and it's not clear how long they are planning to keep the service running. If yesterday's Posterous problems and other Posterous outages over the last six months have you nervous about the long-term viability of Posterous blogs, you might want to consider moving to another blogging platform.
Posterous doesn't have an easy export option, yet (they said in March that one was coming soon). But there are a few ways that you can import your Posterous into Blogger, WordPress, or Tumblr. (Blogger and WordPress would be my first choices). Lifehacker has detailed directions on how to do it. Rather than rehashing what they wrote, I'll encourage you to click through to their directions.