The SMS protocol on iOS previously had an issue that allowed anyone to manipulate the little-used "Reply To" field to make it look like a text message was coming from someone else. iOS 6 fixes the exploit, so the days of pulling that stunt on an iPhone are over.
According to iJailbreak, Twitter user p0sixninja was the first to notice that Apple patched the flaw as part of an "insane number of security fixes" included in the iOS 6 update. The site also says that Android and BlackBerry have not been affected by the issue thanks to more secure messaging apps on their respective platforms. [iJailbreak via Apple Insider via The Verge]