If you're uncomfortable sending a message from your mobile phone in plain text Crypo offers a mobile solution for encrypting and decrypting messages on the go.
The tool is available as a web-based tool sized for both mobile phones and desktop use. You type in your text, supply a password, and hit the encrypt to encrypt your text. Decryption is the same simple steps in reverse.
According to the Crypo web site they are using 1280-bit encryption, although discouragingly they don't specify how they arrive at that value or what encryption scheme they are using. None the less unless you've gone out of the way to anger employees of an acronym-sporting agency, it's quite doubtful anyone will be leveling serious tools at your encrypted missives.
If you have a favourite tool for encrypting mobile communications, let's hear about it in the comments.