http://en.wikipedia.org/wiki/Leap_(computer_worm)
Please note that Leap/A was not only a Trojan, but was reported as viral and propagated using Bonjour. It was also reported in the wild. Also, note that though it's
not yet reported as wild, there are reports just last week of a new piece of OSX malcode that installs with zero user interaction.
I'm relatively sure my friends had POC viral malcode for OSX prior to 2005 in their zoos. Leap/A was discovered in early 2006, so I'm sure the kiddies were playing prior to that- I seem to recall folks looking at Mach-O infection around the 2001-2002 timeframe. I can't say for sure though, as my concentration at the time was Linux malcode, including ext2 viruses, and I found the concept of PPC malcode on the Ford platform a bit more interesting at the time.
The line between self-replicating malcode and ending up on the Wildlist isn't all that wide- it just takes the right timing, incentive or pure luck on the part of an attacker. The fact that Apple continues to half-bury it's head isn't encouraging- for instance, not sandboxing Safari, shipping a known-vulnerable version of Flash with Snow Leopard...
OSX has a lot of the frameworks needed to really be actually secure (most of them coming from the TrustedBSD project originally,) But even though that evolving framework has been built-in for a lot of years, it's not been implemented completely, and it's really too bad Apple isn't being more aggressive, I'd love to see MAC implemented on Macs for instance- the ability to say "This compartment shalt not touch the Internet" would be a fantastic start.
Paul