Spammers are always looking for new ways to target their message that will bypass (at least temporarily) standard spam tracking software.  A recent target was Apple's iCal application through the use of iCalendar invites.

This attack upset many iCal users since declining the invite to delete the invite actually sent a notification to the spammer of the decline, validating the user's e-mail address for use in future attacks. Read more about this and a work-around here: