TXTcryptX is the desktop version of the iphone application for OSX .
If you want to send a sms that only someone knowing a password for it can understand, or just need to encrypt a conversation to hide it for prying eyes, now there is a easy way for that on a iphone.
TXTcrypt works with any character set being unicode compliant, the encryption used is as strong as encryption export regulations allow, not military grade but namely RC4 with keys as big as 64bit depending on the length of your password as such providing a decent amount of security.