It's not in the wild (I love that phrase; I always picture pictographic horses drawn from ones and zeroes trampling through cubicles when I hear "Trojan in the Wild"). In any case, there's a new proof-of-concept Mac Trojan out there that takes advantage of the five minute window after you log in to your Mac when admin-level tasks can be run without re-authenticating. Damien Barrett (that's Mr. Barrett to you!) recommends creating an administrator user for installations, upgrades, etc, and a regular non-admin user that you use for regular day to day computer usage. Good cautious advice.
My two cents: all these proof-of-concepts prove that the concept sucks. I know your 1337 friends all think it's cool that you fragged your Dad's network with your little hand-coded virus, but please, don't make these things anymore, coders.
And for all you virus-fearing Mac users: be cautious, but relax. Nothing is taking down your system any time soon. Also, how funny is this picture?