The flawed systems are SMS and predecessor systems like alphanumeric pagers, where your messages are archived indefinitely in a database by the carrier. Remember the Wikileaks release of all of the pager traffic in lower Manhattan on 9/11?
In my mind, the NSA/PRISM stuff is a meta-problem that is just something that is hanging out in the background. I work in enterprise IT, I'm not a dissident, drug dealer or whistleblower, so I'm more worried about incompetent carriers and other hostile parties than the NSA.
So iMessage gives me encrypted message content that may be readable retrospectively by the NSA, and is almost certainly "tappable" by normal law enforcement.
The reality is, that's about as good as you are going to get outside of a defined community of interest. My employer has highly secure voice and text solutions available for key personnel, for instance. Perfectly secure communications won't be available to the masses, because the operators of those systems face liability.