COOL - Posterous Introduces Seamless Retweeting - The Official Posterous Blog
We've just introduced a new retweet button that makes it super easy to share interesting stuff with your Twitter followers without ever leaving Posterous.
To disable or enable retweeting, visit your manage page at http://posterous.com/manage and click the "Settings Tab". Under the "Look and Feel" section, click "More Theme Options" and check "Enable Retweet Button."
You can also add the retweet button and customize its look on your custom theme. Theme hackers can consult the retweet documentation for our theming engine here.
Big thanks to Backtype for supplying all of the realtime retweet counts.