I seem to have the opposite problem. I am visiting my GF in Australia and I have a US iPhone 4S. She had a 3GS and upgraded to the 4S this week, and ever since, she can't receive my texts. It's weird, because I have iMessage disabled since I am abroad, and she has it active, but she's the one that can't get texts from me (but she's getting them from all her Aussie contacts). I get texts from her just fine.
The problem everyone is talking about happens because the recipient has iMessage turned off, but in this case the sender does. Does anyone have any ideas? Thanks 🙂