It is a myth that is hard to get rid of. No one wants to admit they tweeted out a link to the dev website.

Though I recently found this on the Google+ FAQ: http://support.google.com/webmasters/bin/answer.py?hl=en&...

  When you add the +1 button to a page, Google assumes that
  you want that page to be publicly available and visible in
  Google Search results. As a result, we may fetch and show
  that page even if it is disallowed in robots.txt.

I can understand adding a +1 button to a dev site, and then not understanding why it shows up in the index.