[David Ramsay]

From the US Department of State website regarding the Visa Waiver Programme

Q: Can a VWP applicant for Admission Be Readmitted To the United States Follwing a Short Trip To an Adjacent Island, Canada, or Mexico?

A: Generally, VWP applicants admitted under the VWP may be readmitted to the United States after a departure to Canada or Mexico or adjacent islands for the balance of their original admission period. This is provided they are otherwise admissible and meet all the conditions of the VWP, with the exception of arrival on a signatory carrier, in which case the inspecting officers have the discretion to grant the applicants entirely new periods of admission.
The VWP applicant is admissible and may be readmitted to the United States under the VWP after a departure to Canada or Mexico or adjacent islands provided the person:

Can identify an authorized period of admission that has not expired,
Plans to depart the United States prior to the expiration date of their period of admission,
Presents valid, unexpired passports which reflect admission to the United States under the VWP, and
Continues to meet all criteria set forth in 8 CFR 217 and section 217 of the Immigration and Nationality Act (Act), with the exception of arrival on a signatory carrier.