Question:
"My passport containing a valid visa has expired. I've obtained a new passport; do I need a new visa?"
Answer:
"If the passport in which your visa has been endorsed has expired, the visa in the old passport still can be used, provided that you also carry a valid passport of the same nationality. Note: If, when canceling your old passport, the passport authority has clipped the corners of the passport, and, in so doing, has damaged the visa in any way, that visa is no longer valid and cannot be used for travel to the United States"
Source:
http://london.usembassy.gov/faq-visa-gen.html