I had this problem over the weekend, I upgraded to Leopard and hadn't had my iTouch connected since doing that. My old nano showed up fine but there was nothing with the Touch. After frustrated googling (found this place!) I read endless stories of having to get into my HD - scary thought.

Anyway, I restored the iTouch, still nada and in desperation I reinstalled iTunes. Bingo, it worked. My computer skills really aren't up to much so I was dreading having to fiddle around.

So hello to you all and maybe this might just help another thickie like me.