Glam,
I would not under any circumstances advise depending on pepper spray alone. Well, maybe I'd advise Chuckie Schumer or Al Gore to rely on it...
When I go out, I usually give my pepper spray to whoever I'm out with. I carry a Marlin Guide Gun in .45/70 stoked with Garrett Hammerheads. The plan being that if a situation arises, I'll cover while the other guy sprays the bear.
You have no idea how hard these animals are to kill when they are adrenalized! In this situation, the bear is by definition, wired, or you wouldn't be considering shooting him in the first place. I can relate many first-hand accounts of large brown/grizzly bears taking multiple hits from .338/.375 class weapons to bring down. You don't want to start shooting at close range when he's already considering attacking.
As for your question about whether a bear ever attacks following a threat display (when the human stands his ground and plays smart)... Well, yeah, it does happen sometimes, but it really is quite rare.
As for bears vs humans and pepper spray - a human relies on reasoning at some level. He doesn't rely on sight, smell or other sensory input - if he's determined enough and figures he can get close enough to grapple, he'll ignore the pain and do so.
Animals are different, cut off the sensory input (along with inflicting pain) and they'll normally retreat.
All of this depends on using some basic common sense. For example if you were to squirt a sow and cubs, you might just create an attack the moment those cubs start bawling in pain! Use it on a windy day and let it blow back on you and you'll be on the ground crying and acting like "wounded prey". Not good.
I've talked to many mauling victims and read many hundreds of accounts of maulings. There are many, many cases of people shooting a bear and then getting mauled for their trouble. Its a very common scenario. I have yet to read of an account where somebody sprayed a bear and then got mauled. It may have happened - it surely will some day, but your odds are better with spray than with a gun - if you use it in the right time and place.
------------------
Keith
The Bears and Bear Maulings Page:
members.xoom.com/keithrogan