As of right now, when someone Tweets a link to a SlideShare presentation, document or video — it is embedded on Twitter.
We’re excited to partner with Twitter on this new capability. We’ve always made speed an important focus for the SlideShare experience and enabled embedding broadly across the web. This should enhance the experience on #newtwitter and increase the number of views for content shared on SlideShare as it goes viral on Twitter.
You can see the announcement on the Twitter blog here: http://blog.twitter.com/2010/12/now-playing-on-newtwitter-embedded.html