CryptoGraf is a tiny company that is basically selling RSA/ARS encrypting SMS software. You can try a free trial at CryptoGraff.com and it works with Symbian Series 60 phones but it doesn’t work with any of Nokia’s N-Series phones.
Why am I telling you this? Well, maybe you have to send secrets to your mom or something. The website is gimp-tastic but it looks like a useful tool.