I never ran an autoblog, but I watched several. For a long time they could stay ahead of Google penalties if they added enough posts every day. That is because the duplicate content filter was run separate from the ranking algorithms, so there was always a lag of weeks or months before a post was flagged for duplicate content. Continually adding new content kept the sites ahead of the wave.
From what I have seen in the post-Panda environment, once a site reaches an unknown threshold number of infractions, it no longer ranks well and Google will eventually de-index the site.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin