Apple has patented a system for blocking incoming or outgoing messages based on their text content, which could potentially be used to prevent "sexting" or sexually-explicit messages being sent or received on future iPhones. The patent, "Text-based communication control for personal communication device" (7814163) suggests "defined criteria" such as from preset parental controls could be used to instruct the censor, which would either refuse to send the offending message or strip out the non-approved content beforehand.
"Systems, devices, and methods are provided for enabling a user to control the content of text-based messages sent to or received from an administered device. In some embodiments, a message will be blocked (incoming or outgoing) if the message includes forbidden content. In other embodiments, the objectionable content is removed from the message prior to transmission or as part of the receiving process. The content of such a message is controlled by filtering the message based on defined criteria. The criteria may be defined according to a parental control application. These techniques also may be used, in accordance with instructional embodiments, to require the administered devices to include certain text in messages. These embodiments might, for example, require that a certain number of Spanish words per day be included in e-mails for a child learning Spanish."
Apple's patent documentation doesn't just deal with "users such as children [who] may send or receive messages (intentionally or not) with parentally objectionable language," but education and grammatical uses too. For instance, the iPhone could refuse to send a message that fails on spelling and/or grammar rules.
However, it's more complex than a simple blacklist of dirty words. Apple describes the ways - alternative spellings, abbreviations, etc - by which users can bypass such restrictions, and suggests instead that their content control would be "intelligent" to catch out users trying to game the system.