The first virus for Mac OS X has been found today. It is called OSX/Leap.A.

The malware was originally posted via link to MacRumors forum pretending to be screenshot for Mac OS X v10.5 Leopard.

Update by Gergo:

Leap.A propagates through iChat and infects local applications using the companion method. Our description is located at

https://www.f-secure.com/v-descs/leap_a.shtml