It's for domestic business class passengers.
Is it really this simple?
Must present boarding pass or e-ticket.

I tried a saturday night and didn't appear to be available. Sunday night the offer was there. It looks like you have to book a room at the regular price. A pop-up screen says free night will be accorded upon checkout.

www.panpacific.com/aircanada

We invite Air Canada Business Class travelers to enjoy a complimentary night's stay between October 1st, 2008 and January 31st, 2009.

Simply present your boarding pass or a copy of your electronic ticket at check-in, then relax and enjoy our idea of a window seat.
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Terms & Conditions
Offer valid for stays booked from 1st October, 2008 until 31st January, 2009.
Proof of Air Canada Domestic Business Class Boarding Pass must be presented upon check in for complimentary night stay.
Bookings to be made via panpacific.com website.
Advanced booking is required to redeem the Complimentary night (room only) offer.
Promotion is limited to 1 complimentary room night in our Deluxe City view guestroom. Room taxes, gratuities, incidentals & parking not included.
Only one complimentary night per business class traveler may be redeemed during the promotion time frame of October 1st, 2008 through January 31st, 2009.
Blackout periods may apply. Offer subject to availability.
Accommodation is not guaranteed until a Reservations specialist contacts you. Please provide the best telephone number you can be reached at during business hours.
To speak to a Reservations Specialist, please call our toll free number at 1 800 663 1515.