The "Mp3-Concept" trojan is no threat. The main thing it exposes is that there is no way for it to propagate or transfer over networks, because the resource fork is normally stripped out in any kind of transfer. Most Mac users are aware that you don't convert an mp3 file to mp3.sit, mp3.zip or mp3.hqx in order to transfer it, so such a trojan could only affect the most uninformed newbies, who are always the guinea pigs for any virus/worm/trojan in any platform.