I was looking for a way to encrypt calls and sms on Nokia N900 and i only found ways to send sms and make a call from CLI.
It would be great if you could share encryption keys by bluetooth (or write them manualy) and add them to an "Encryption key" entry in the adressbook and then use that key (or write an new key) when you send sms and make calls and encrypt it.
The encryption would be optional and you can turn it on/of by an menu entry in the sms/call application where you also can chose if you want to use the adressbook key or use an new key and the sms/call will be encrypted until you turn it of.
The encrypter would put for example #ENCRYPTED# in the end of the message, in that way the phone knows what messages it should decrypt.
The encryption/decryption part i dont think would be so hard but how to get the unencrypted data to the encrypter and then send it?
And how to add the things needed to the menus in the call/sms application?

You can encrypt almost anything with truecrypt. Here is a tutorial for encrypting contacts and messages.

I want so send it encrypted Not just encrypt it on the device

Have you considered somthing based on the OTR protocol?