http://travel.state.gov/travel/cis_pa_tw/cis/cis_1759.html

Would-be applicants who do not need a visa to remain in the United States may find it more convenient to apply for a visa elsewhere in conjunction with their next foreign travel. Those who plan to visit Canada, Mexico or, in the cases of students and exchange visitors, adjacent islands, may re-enter the U.S. within thirty days on expired visas as long as they possess a valid I-94 form unless they are excluded from automatic revalidation, as noted below.