With the move from the Goods Services Tax (GST) to the Harmonized Sales Tax (HST) in the provinces of Ontario and British Columbia, business owners and operators may have many questions about how the new HST applies to transactions that straddle provincial and territorial boundaries. In these situations, place-of-supply rules will be applied in order to determine the province in which a given supply is considered to have been made, for GST/HST purposes. The new place-of-supply rules came into effect on May 1st, 2010, and apply across Canada.

Here's the complete Government of Canada document.