Reports are coming in of a number of Skype users getting calls via Skype telling them that they have viruses. Interestingly, many of the people getting these calls are Mac users, and, when the callers tell them that they have Windows viruses, they are right to doubt them.
Many Twitter users are posting about this, and it seems like a good idea to point out that this is obviously a scam. The malicious users try to send their victims to a scam anti-virus website, which claims to sell both Windows and Mac antivirus software.
UPDATE: here's what the calls sound like. From http://krebsonsecurity.com.
To ensure that you don't receive calls from people who are not in your contact list, open the program's preferences, click the Privacy tab, and change all the settings from Anyone to Contacts.
So, if you get a call like this, hang up. There's no way a remote user can know if your Mac is infected, and in no way would any serious security vendor cold-call people trying to trick them into buying anti-malware software in this way. (Naturally, if you do want to be protected, you can check out Intego VirusBarrier X6 which provides full malware and network protection for Macs.)