What's that ball doing in your can of Guinness? Well, Tory of Hobson's Choice blog found out the answer:
Basically, the small plastic ball is called a widget. The purpose of the widget is to release CO2 from some of the beer in the can to create the head. It is meant to mimic the texture and head of Guinness on tap.
Mystery solved! http://www.toryhobson.com/new/entries/not-your-macs-widget/ | Longer explanation at How Stuff Works - Thanks Tory!