The first Trojan targeting Mac OS X (OSX/Oompa-A) is in the wild but seems it’s not that critical according to this site.
You cannot be infected by this unless you do all of the following:
1) Are somehow sent (via email, iChat, etc.) or download the “latestpics.tgz” file
2) Double-click on the file to decompress it
3) Double-click on the resulting file to “open” it
…and then for non-Admin users, it fails to infect most applications.
Most of the antivirus companies have released updates to detect the threat and here is Safety tips from Apple for Mac users.