Ausgirl - the books suggests if the child is over 22 months and can communicate (not necessarily with speech, but even with gestures, behaviours, sounds, signals) then they are ready

Actual interest in the toilet or wanting the nappy off etc aren't important, its more about being able to read the signs of when the child needs to use the loo. The theory is that kids will mimic/copy as part of natural development and going to the toilet is no different that any other thing they pick up/learn from older kids/parents