I think it depends on the level of numbness. I've been barefooting in winter on and off for most of my adult life (I say "on and off" because I've spent about half of my adult life in warmer climes). I do stuff like grill outside, shovel the walk, get the mail, and go on short walks. Until last week, I've never had a problem with frost bite/nip, even though the feet invariably get a little numb when it's below 40F /5C or so, or even in the 40s if you're not moving around much.
I think there are three signs your numbing up number is up:
1), if your feet keep getting number and number even after one or two miles or 10-15 minutes of running, or if your feet numb up much more quickly than usual. Usually after a mile or so my feet-heat kicks in and the numb-plunge is stabilized or reversed, depending on temps/moisture/wind;
2), if your feet start to become stiff; and
3), if you get home and the feet tingle painfully.
Up until the week before last, I only had one instance of coming home and having the feet tingle painfully a bit, and that's over one and half winters of barefoot running. Even so the pain was very mild and only lasted a few minutes. Usually I just wait 10-15 minutes for the feet to warm up sufficiently so that the hot water of the shower isn't painful, but they aren't painful while they're warming up (but don't stand next to a heater or anything).
On my frost-nip mishap 10 days ago, I knew I should've started out shod until the feet-heat had kicked in, but I hadn't needed to put on shoes up until that day's run, and so I was stuck on the idea of trying to finish up 2012 purely bare. Fail. The feet numbed up really quickly, which was my sign that I was in over my head in the half inch of freshly fallen powdery snow.
As for which toes, I'm like you, my index toe is most sensitive, and my middle toe next. And those are the ones I noticed were white when I got frost nip. The other ones were still red at that point. Even last winter the very tip of one of my index toes got a little nip that took a few months to go away.
As for expert opinion, I think anyone who gets frost bite more than once or twice should get their license revoked. An expert understands their limits and does not exceed them. If you think it's neat to post a collection of photos showing all the times you've been frost bitten, you're more eccentric than expert, in my opinion.
And you can definitely get frost bite above freezing. Frost bite depends on body temperature, not air temperature. Our bodies are close to 100 F. When tissue becomes significantly colder than this for a certain amount of time, it can become damaged. Usually our circulation keeps this from happening, but if it can't keep up and counteract whatever it is that is cooling the skin (temp, wind, moisture, capillary constriction in order to maintain core body temp), damage can happen. And it doesn't take very long for it to occur. On my frostnip mishap I covered up after just 10 minutes or so, but it was already too late (it probably didn't help that I didn't bring socks along, so my feet were still exposed to relatively cold temps even once inside my shoes).
Interestingly, people who habitually work with their hands in the cold, like fishermen, do develop a mechanism for flushing their hands' capillaries with warm blood every 7 to 10 minutes. So adaptation, to some degree, is possible.
And be warned that Zetti's advice about women and bare feet may not be generalizable. He married a Brazilian, and they're all crazy (but in a good kind of way).