According to the US, if a company is registered in the US, all data they hold anywhere in the world is fair game. So this includes all servers used by Microsoft, Amazon and Google anywhere in the world according to the US. And in the last few days there has been another example of this:
"Google is the next major company to admit Patriot Act issues, as it admits to handing over data held in a European datacenter back to U.S. intelligence." ZDNET, August 11, 2011
I am not a lawyer and not related to Ubuntu One/Canonical and not a lot of users here are legal experts with knowledge about this situation so an answer to your question will all be conjecture. And it also depends on how stubborn the EU is about enforcing own law and opposing the US Patriot Act.
Ubuntu One Terms of Services
This agreement is governed by the laws of England and any dispute will be heard by the courts in England. Failure by Canonical to enforce any right or provision of this agreement shall not constitute a waiver of such right or provision. If any part of this agreement is held invalid or unenforceable, that part will be construed to reflect the partie's original intent, and the remaining portions will remain in full force and effect. The terms of this agreement do not affect your statutory rights.
Ubuntu One is based in the UK so the company is subject to the EU laws and the
European Data Protection Directive. So the answer to your question should be
no Ubuntu one is not affected by this if you store the data on a EU server
Any data stored on a cloud worth protecting should be encrypted. Might not stop the US from getting it and also might not stop the US from decrypting it but it should be made as problematic as possible ;-)