Over one million users were recently impacted by the Flash based worm stored on MySpace pages.  The site is now requiring the latest version of Flash to prevent future occurrences. 

MySpace Worm Attack - Analysis by ISC
http://www.incidents.org/diary.php?storyid=1510
http://www.avertlabs.com/research/blog/?p=59
http://forums.mcafeehelp.com/viewtopic.php?t=83945

QUOTE: An unusual aspect of this worm was that it resided purely on MySpace pages, rather than installing itself on personal computers of its victims. The essential component of the worm, which Symantec called ACTS.Spaceflash, was a Flash object that was embedded in the victims' profile pages on MySpace. The offending code resided in the redirect.swf file