Two weeks ago, Second Life users were warned of a flaw in QuickTime that allowed carefully crafted QuickTime datastreams and file headers to access their accounts through the viewer, and could potentially be used to steal items and Linden Dollars. The flaw was in QuickTime itself and was usable to compromise a wide variety of software unless you disabled or uninstalled QuickTime to prevent it running.
A few hours ago, Apple finally dropped a new version of QuickTime (version 7.3.1) which fixes these exploit issues. Whether you use Second Life or not, if you have QuickTime installed on your machine you should get the update without delay.