Fortress SMS is a phone based application
to send and read encrypted SMS text messages.
The messages are encrypted prior to transmission
and can only be read by the intended recipient on
entry of the correct password.

Trial version available (fully functional reader
but only allows up to 3 characters in the password
for outgoing messages). Although the password is
processed using standard cryptographic
techniques prior to encryption, sending messages
using the trial version should NOT be considered
secure.

http://www.fortressmail.net/fortress_sms.htm