Well, isn't this intriguing. In a boon for privacy and civil rights advocates, the federal government recently discovered that they can't actually collect text messages from people they're wiretapping if they own an iPhone and are texting someone else with an iPhone via iMessage. As CNN reports, an internal memo circulated just last month amongst the DEA discusses how iMessages are hardware encrypted, and are not being collected in any readable fashion with other text message data.
This means they would need someone at Apple itself to cooperate with them, should they need to decrypt some messages, or they would need to break into a suspect's physical phone hardware.
Texting has been frequently at the center of court cases during the last decade in challenges to the federal wiretapping law. The Federal Wiretap Act was first passed in 1968 and updated in 1986, but there remain tons of gray areas surrounding computer communications and texts. Essentially, though, gaining access to a person's texts via a telephone service provider falls under the provider exception, in which law enforcement officers with a valid court order can direct the service provider to hand over this data.
iMessage has been around since 2011, but it seems the feds are only recently catching on that it inhibits their ability to access text data.
Update: Well, it seems that this memo may have been purposely leaked by the DEA, and it remains ever possible that the feds could go to Apple directly to gain access to the messages. As TechDirt adds, "in fact, in many ways iMessages may be even more prone to surveillance, since SMS messages are only stored on mobile operators' servers for a brief time, whereas iMessages appear to be stored by Apple indefinitely." [TechDirt]