While you have to leave network security to the pros, you can make certain your mobile communications are secure with Open Whisper Systems' open-source RedPhone app. The app makes super-encrypted calls to other RedPhone users over a standard data connection. The coolest part? It works with your existing contacts and dialer, and there is no need for PINs or other identifiers.
The companion app, TextSecure, provides the same level of encryption to your text messages.