Welcme to the Frums!
Make sure your data plan has been added to your service.
Is it a picture message or a text message that you are getting the red X?
Lastly, who is your service provider?
I am also having the same problem. I cant send text on my 8900 curve it says unidentified subscriber. I dont know if it is because i upgraded to 5.0. pls someone help!