From Taser Weapons at http://www.taser.org/faq.html:

"Will this thing stop a grizzly? I have three small children under 5. We backpack the Cascades, 4X4 camp in remote areas. My wife and I both pack high caliber automatic handguns as
defense against a bear or human predators. (We ran accross an mom a few years back and narrowly escaped with our lives.) We would like to get rid of the handguns with the accompanying dangers to our children and carry a non lethal weapon such as a taser. I have used a taser against a human attacker with great success. How effective is this device against a blubbery grizzly? Will it put her out long enough for us to withdraw?
Any help would be appreciated."

"We absolutely cannot guarantee that it will have any affect
on a grizzly. We have no record of anyone ever having
used a TASER® Weapon in the wild on a grizzly. The TASER® Weapon is designed to work on a human, and therefore we strongly advise you not to carry it as your only means of defense against a grizzly or any other type of animal. When used against a 2000 lb buffalo it stunned him for only an instant. He dropped onto
his hind quarters, but was back up in 2 seconds and became
very angry. If a grizzly reacts in the same way, it would make
a bad situation even worse. Others do carry TASER® Weapons when
in remote areas for protection against human attackers, but
keep your firearms for protection against wild animals."

Sue