I don't think that it has so much to do with what they wear on their butt - but whether they comprehend the actual feeling of needing to go to the toilet and then linking that feeling to the toilet.

DS has only just started now to not scream at the potty, and will sit on it, but with a nappy on.

I'm not going to force my child to use a potty/toilet earlier than he is ready to. I've heard that if you start early, and the kid ain't ready - its a whole lot more hassle and takes longer than if you just start it when they are ready.

JMO of course