The first of a kind worm that was specifically targeting Skype users has moved on over to greener pastures, and is now aiming at instant message networks. It uses pretty standard tactics for other IM worms, using infected machines to send messages to people attempting to get them to download a .pif . Of course, opening said file will result in compromise on a Windows machine. It seems the worm will look for many different clients, making it somewhat unique. It will scan and try to send messages through MSN, AIM, Trillian, YIM, Miranda and ICQ, though the article mentions that it has been seen only sending through MSN and ICQ:

"The infection checks the registry for evidence of programs like AIM, Trillian, Yahoo Messenger, Miranda and ICQ - however, so far we've only seen it fire a message to an ICQ and an MSN Messenger Client," writes Chris Boyd, director of malware research at FaceTime. "The main target appears to be Skype with regards a delivery mechanism for the messages sent, but the potential for the infection to leap across various networks is obviously there."
As always, thinking before you click is the best defense here, and suspicious links are not hard to spot.