I recently had to write a script for an app that polled a blog to retrieve content from the latest post. While blog updates are communicated using Atom/RSS feeds, these feeds by default only provide access to the last X items from the blog. The trick to retrieving earlier content is to append ?max-results=1000 to the feed URL to, for example, retrieve the last 1000 posts. I'm sure there's a limit to this number, but it was sufficient in my case.
Hope this helps someone out there.