This is Serious!: The Next Big Virus Threat: Instant Messaging

"Symantec, makers of Norton Antivirus, says the next big worm is likely to exploit flaws in instant messaging clients�and it will spread faster than any other worm on record.
'Code Red was able infect every vulnerable machine on the Internet in 14 hours. Slammer was able to do it in 20 minutes,' says Eric Chien, chief of research at Symantec Security Response. 'An instant-messaging threat could spread to a half a million machines in 30 or 35 seconds.'"

PC Magazine, June 23, 2004

This could be a Real Problem I think we should keep an eye on this