回複：大牛小牛請進，關於中國與美國，英國與美國的退稅協議

The treaty(article 19) says "An individual who is, or immediately before visiting a contracting state was, a resident of the other contracting state". So it doesn't matter where you came from, what matters is your nationality.

Check on www.irs.gov, search for "tax treaties", following is the website:
http://www.irs.gov/businesses/international/article/0,,id=96739,00.html