As I understand it, there are two issues with walkers. One is that there have been many incidences of kids falling down stairs etc. In fact Choice did a study a while ago on the top ten most dangerous products for kids and both walkers and JJs were right up the top of the list. Interestingly enough toy boxes were also on the list(fingers caught in the lid I think was the main injury).

But the other thing with walkers is that they are putting kids in a position they are not ready to be in (ie standing with feet on the floor). I have read in a few places that this can cause issues down the track with the child, although I can't actually remember the exact problem. I'll have another read and post an update if I remember.