A new Mac OS X Trojan horse has been getting some the-sky-is-falling press lately, dubbed MusMinim by Sophos, and know as BlackHole RAT elsewhere. If you believe the hype, this one is as bad as they get, including the ability take remote control of the Mac and redirect web traffic.
I did some research, and this Trojan is just a proof of concept written by a German kid, and poses absolutely no risk to the public. To become “infected” you have manually download it from the guy’s website, and enter an activation code to install it (in addition to authenticating as an admin user, as with any Trojan install). I personally would not even classify this as a Trojan.