@Kaleb: The purpose of a Ursack is NOT to prevent bears from reaching your food. The purpose of a Ursack is to provide a means of preventing your food from being carried off. It will not protect your food from getting crushed and your food will have smelly bear saliva on it if / when they reach it as it is tied to an object that the bear cannot cart the Ursack away from.
If you hang your Ursack, you've bypassed it's primary purpose because if a bear reaches your food, it will go with them as they skip out of camp. In problem areas, bear hanging may not work because as others have mentioned, if it always worked, we wouldn't need bear cans, but we do.
Bear cans cannot be carried off, so just like a Ursack, we don't hang them, but because their size prevents a bear mouth from carrying them away, we don't have to tie them down. A Ursack does fit into a bear's mouth and can be carried away, so securing the ursack to a tree or simlar prevents a Ursack from being carried away, some will disagree with even this though.
Hanging Ursack = potential for food being carried away
Wedging Ursack in high rock = potential for food being carried away (if you can reach it, a bear likely can and high rocks aren't easy to find, probably not as easy as finding a tree and following ursack instructions and using it correctly)
hiding Ursack = potential for carrying away
Tieing Ursack to tree or other object per Ursack instructions = most likely way to prevent food from being carried away.