There have been some early reports today of a Mac Trojan Horse, sparked by a press release at Intego. This MacNN article by David Schroeder provides details of how it would work and seems to clarify that this is a possible security problem rather than an actual trojan which could pose a current threat.
I don’t think there’s ever been any cause for Mac users to be complacent, so this potential source of concern shouldn’t be causing alarm. Sensible precautions are to treat any file as a potential problem, including expected attachments from known and otherwise trusted sources. No-one with any clues at all should open unexpected attachments from unknown sources.
Update on 10 April: for further discussion on this “trojan” see the TidBITS Talk discussion.