I have heard that somebody else can get access to your phone's messages by registering your phone in the provider's website. If that person has all the details and info needed, he/she can track your phone details. For cingular, this is the website:
https://www.myaccount.cingular.com/
Try to check if somebody has registered your phone. If none, then you must be happy coz if it is already registered, then somebody must have tracked your phone and he/she can track your messages even if she/he is at the other side of the world.