cloth diapers are an incentive...
to potty train (wet = yucky) that's for sure, but as far as making kids 'learn' faster, I doubt that. I also agree that when kids where cloth diapers, they're more aware of what's going on down there.
However, I figure the reason kids used to potty train earlier was b/c parents began potty training them earlier. The notion of 'wait until your child can verbally tell you s/he has to go' or 'wait until your child can physically pull up/down his/her pants' is a new concept. 30 years ago, kids potty trained earlier b/c parents introduced the potty earlier. And 30 years before that, kids potty trained even earlier b/c their parents introduced the potty earlier.
As you've probably guessed, I'm an advocate of infant potty training. Most ppl are skeptical, even my 'crunchy' friends, but it's gone swimmingly for us. We started with DD#1 at 13 months, she was dry through the day at 19 months. I started with DD#2 at 2 wks, she was napping diaperless at 4 months, and for a patch, we had days where we'd use one diaper, b/c we went out. Of course, we have had lots of accidents, but those are usually b/c I'm being inattentive.
We're having a rough patch now b/c she just became mobile (11 months, crawling, furniture walking, climbing up stairs), and she's lost interest. I'll get 2-3 pees from her a day, but it's a struggle, and I don't force her. It's been a neat learning experience though, I was quite skeptical when I tried it with DD#2 so young, and to see her progress was a great incentive to keep it up.