From Daniel Eran Dilger Hacker discovers iPhone SMS spoofing issue, asks Apple to fix for iOS 6:
An independent security researcher in the UK has publicized an iPhone SMS spoofing issue that he hopes Apple will address in iOS 6.
According to a blog posting by "pod2g" the way iOS handles SMS messages supports transmission of optional, advanced features in the SMS specification's User Data Header, including a "reply to" address.
Not all phones support these features, and "most carriers don't check this part of the message, which means one can write whatever he wants in this section," the hacker writes. This would apparently limit the audience of SMS spoofing largely to iPhone users.