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