Just tried a test booking for YYC-HKG, and got the option to hold until tomorrow at 10 PM MST. This is a great option to have online!
Here's the text:
I Prefer to hold this booking until November 08, 2009 at 21:55 PM Calgary time.
If the ticket is not purchased before the deadline date and time, the booking will be cancelled.
Only one booking may be kept on hold at one time.
Once a booking is put on hold, any À la carte options you may have selected will not be saved. You may select À la carte options after ticket purchase, however discounts options will not be available at that point.