IOS Jailbreak How do intercept SMS / Text Messages

I'm currently trying to write an application that intercepts text messages and reacts depending on the content of that message. I tried to hook into _receivedMessage:(struct __CKSMSRecord *)message replace:(BOOL)replace method in the CKSMSService class but this seems not do get called at all.

Could someone please tell me what function/class i have to hook in? I need to intercept the text message before it gets displayed and stored into the database. I'm on IOS 5.0.1.

Any help is truly appreciated.