This is a subject that comes up here with some regularity. The current issue of Handloader magazine (Oct. 2001) has a couple articles on the subject.
Once you get past the caveat that you really want a big rifle or a shotgun with slugs, then it's an acknowledgement that even a small gun is better than shoving your arm down the bear's throat and hoping he chokes to death.
The author of the main article, Brian Pearce cites some interesting emergency uses of a handgun on bear. Some were successful, some weren't. He makes some points that make a lot of sense, and some that don't. He makes the point that most commercial jacketed pistol ammo is designed to expand fully in just a few inches and that for bear you want 15 or 20 inches of penetration. He invokes Elmer Kieth and the use of a hard cast lead bullet the heavier and hotter loader the better. All this makes sense to me. I don't agree with his outright rejection of autoloaders. Sounds like six-gun snobbery to me.
I have loaded 250 gr. bullets in .45 ACP for bowling pin matches, and it is surprising what a difference the extra 10% of bullet weight makes. It wouldn't be my first choice for bear, but if the .45 is all you have, then better to have the heavier bullet loaded as hot as you can stand it.
Reading the stories of bear encounters was fun. The story of "Old Groaner" was worth the price of the magazine. I have had the willies about encountering a grizzly ever since I read the diaries of Lewis and Clark. How stealthy and fast bears can be is a marvel. One story is a guy who clocks a bear by driving behind as the bear runs down the road (25 mph for a goodly distance).
The interesting thing was that while there have been some cases where a handgun has saved the life of someone in a bear encounter, most always the person gets mauled or chewed on some.
If you've never read Handloader, pick up a copy. It always has something of interest.