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

"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