According to Eric Chien, chief researcher at Symantec Security Response in Dublin, Ireland there are 60 published IM vulnerabilities and over 30 identified IM worms. These run the gamut from holes that allow the IM client to be crashed to vulnerabilities that allow malicious code to be installed on the user's machine. Because IM Worms can hijack the buddylist, they can propagate quickly. This article from PCWorld.com has more detail on IM vulnerabilities.