I've taught my first two kids to ride the week of their 3rd birthday. The method I used was to keep raising the training wheels until they weren't ever touching. Once he or she gets comfy with a given height, give them a week or two, and raise them 1 cm (bend the arms or re-drill holes if you have to).
Then, have a $15 craiglist bike handy w/out training wheels. Let them try it w/out the trauma of seeing their own bike w/out the training wheels.
Also, offer a kit for learning to ride w/out training wheels. You have to have a good shop around to have shorts and jersey that fit a skinny 3-year-old girl...
My goal is to get my youngest off training wheels before he turns 3, but I'll let him determine if that's going to happen... as it is now, he LOVES trikes, bikes, and scooters -- so the interest is there.