Dan,
The bottom line is that the people who have touted the .45 (or any other handgun caliber) as a spectacular "stopper" have been doing so without any hard evidence for their position. Because many of these people are in positions that render them trustworthy to the gun community, their OPINIONS have long been accepted as fact.
We're seeing in Iraq that even a torso hit from a centerfire rifle doesn't guarantee a stop--not even MULTIPLE hits. That kind of puts quibbling about the minor differences in handgun calibers into perspective.
Given that not even a rifle bullet has the oomph to take a person down with a less than perfect hit, it doesn't make sense to pick a handgun caliber under the false impression that a slightly larger bullet will make a difference. Go for capacity, go for shootability, go for minimum recoil recovery time. If you can make good center-of-mass hits, do it repeatedly and really fast you will have a much better chance of surviving. Anything that cuts down your speed, your accuracy or the number of shots you can take is not a good thing and could cost you your life.
I'm not going to set a caliber minimum, but I will reiterate that it is important to shoot something that has the capability to penetrate through and through on an ideal shot (straight on shot) made on the average human. That's important because you may not get an ideal shot and you will need that penetration to get to the vitals if you don't. I don't think rimfires are a good idea because they tend to be less reliable than centerfire.