OSX/Leap.A is a Trojan/worm that targets Apple's OS X operating system, and is the first to be found in the wild. (Versions of Mac OS had their own malware before Apple switched to Unix.) Users are offered a file called latestpics.tgz, which is claimed to include pictures of the next version of OS X 10.5 (Leopard). If you unpack it, there's a script that pretends to be a JPEG file. Clicking that runs the script (which may well require you to enter your administrator password), and the worm tries to propagate itself via iChat's buddy list.
Clearly OSX/Leap.A requires a lot of user intervention, but this kind of social engineering can work. At least one Windows virus arrived in a password protected zip file but users still managed to extract and run it to infect their PCs.
There's a good account on the Ambrosia Software Web Board, which notes:
It seems that this is more of a "proof of concept" implementation that could be utilized to actually do something in the future, depending on how successful it is, or it was simply done to garner attention/press. Which I'm sure it'll get.
See also F-Secure's Web site and Sophos.
