Just a few minutes ago Apple released security update 2006-001, in particular the update is described as fixing holes that the Leap.A trojan exploited.
iChat. A malicious application named Leap.A that attempts to propagate using iChat has been detected. With this update for Mac OS X v10.4.5 and Mac OS X Server v10.4.5, iChat now uses Download Validation to warn of unknown or unsafe file types during file transfers.