I found some very interesting info over at Warriortalk, posted by some folks who I respect and who are well versed on ballistics.
It seems the math has been done, and the .45 has a 4-7% probability (per shot, depending on whether or not it expands) of hitting something vital (spine or major artery) that a slightly smaller 9mm barely misses. That means if you shoot 100 rounds, you would nick something vital with a .45 that would be a near miss with a 9mm only 4-7 times on average. You are better served by firing more 9mm rounds - this gives you a much greater chance of hitting something vital than relying on the larger size of the .45.
This small per-shot advantage for the .45 comes at the expense of less magazine capacity (in a given size gun), greater recoil/slower follow-up shots (all else being equal), and more expensive practice ammo.
Ammo capacity is important to me, as I do not expect one shot from any handgun (regardless of caliber) to stop a determined attacker. Thus, controllability and the ability to make fast, accurate follow-up shots is also important to me. The 9mm has a distinct advantage here.
For me, the small per-shot advantage of the .45 over the 9mm is not worth the associated trade-offs. By all means, if you can conceal-carry a double stack .45, and make repeated hits with it as quickly and accurately as you could with a 9mm, go for it. Most folks, however, would likely find that hard to do.
So, I'm sticking with the 9mm caliber. I've conceded that there must be some per-shot advantage to the larger calibers, but I've been unconvinced that the advantage was great enough to warrant the associated systemic trade-offs. Now that I know the actual "number" I have even more confidence that the 9mm is the best all-around caliber for self-defense for my purposes.