Eavesdropping on your mobile is more widespread than we think, as proved by the recent News of the World phone hacking scandal. WISeSMS allows you to choose whether to encrypt an important text message before sending, but it also works with emails and simple chat sessions as well.
The user on the other side just has to use a password you've given them to decrypt the message for reading. WISeKey's solution uses your native messaging or email apps, and secures it with AES symmetric-key encryption. Moreover, you can change the password for each message to up the level of security. There is also a web-based version if you don't want to install the native app.