The only knife I'd trust to do in a grizzly would be one of those Wasp injection knives. Just plunge it in and push the button. I've done some reading on this knife and it's supposed to work well with sharks. Then, recently, I read James Rollins'
The Doomsday Key, in which the Wasp was featured.
I also saw on the National Geographic Channel (or one of those networks), where a man saved his girlfriend from a black bear using a 4-inch knife. He had some bumps and scrapes, but came out of it better than the bear.
Knives are mean weapons, silent and effective. I don't want to go up against a bear with one, though. I saw a movie tonight called
The Edge, starring Alec Baldwin and Anthony Hopkins. They were stalked by a grizzly and eventually killed it. They weren't too smart, though. They trapped a squirrel and were very happy, then went off and left it when a helicopter failed to spot them.
I had a girlfriend like that....