I wonder how much credibility there is is stuff like this, or are they just trying to create more fear-factor and sell stuff. This was forwarded to me by a friend:

From: PC Tools
Sent: Thursday, April 16, 2009 10:57:29 PM
Subject: Important Notice - Mac Threat Alert

Mac Threat Alert
Know anyone that's a Mac User?

Then let them know that malware researchers have discovered what appears to be the first Mac OSX botnet, aka MacBot or iBotnet and its receiving a substantial amount of industry and media interest.

Early estimates suggest that thousands of people have unknowingly downloaded the infected files. Don't let yourself be one of them...

In a nutshell the botnet is launching a number of malware variants inside pirated copies of popular Mac based software in order to take control of the infected Mac machine.

First and foremost make sure that you and your fellow Mac users, exercise caution, have Mac specific security software installed and that your existing version of iAntiVirus is up to date - there's a free version or you can purchase a version with full functionality and support for only $29.95. It's also vital that you enable and install the latest Mac security updates.

Regards
PC Tools Team