It's not secure, but you have a reasonable expectation of privacy using text messages, at least as long as you trust the other person. Not "get away with crimes" privacy, but "I'm getting so-and-so this for their birthday" privacy.
Then my friend tells me hes getting my text messages. Every one of them. So I get the phone back, call telus up and complain. The tech has me enter some number which opens up the config for the phone.
And low and behold I have my phone number sitting in the phone. Deleting the number stopped the text messages.
So lets think about this for a second. All I have to do to get your text messages is enter your 10 digit number into my old non active phone.
I would say security in text messaging is non existant.
FYI: Happened christmas of 2009 so it might be fixed but I doubt it.