iPhone users can thank two researches Collin Mulliner (a Ph.D. student at the Technical University of Berlin) and Charlie Miller (Security Researcher) who presented at a Black Hat Security Conference in Las Vegas that they could take control over an iPhone merely by sending a SMS message. It was described by CNET as a serious memory corruption bug in the way the iPhone handles SMS messages.
Although there are no reported incidents of users phones being affected by this security issue, I would recommend all iPhone users upgrade as soon as possible. Unfortunately, it will take a little less than an hour including the backup time, much less if you don't have much data on your iPhone. Before you upgrade, make sure you have the lastest version of iTunes.
I upgraded several phones this evening on both my Windows PCs and Macs without any issues. Thanks to my friend Jose A. who alerted me of this release via text message. I am now subscribed to the Apple Dev Team's Twitter account @iPhone_Dev so that I can monitor these types of issues.