To me this sounds like some kind of SIM provisioing error, but just to rule other things out I have some questions:

1. Do you have any MMS apps installed on you N900?
2. Have you checked the contents of the contact in you phone book?
3. Have you cleared any SMS history in use in both devices (I'd clear one then test then clear the other)?
4. Are there any special charaters in the SMS that?