John Benitez is incorrect about the affect on the batteries as the chargers are "Smart" chargers and will use only that voltage and amperage that is needed to charge the unit. Therefore the iPod or iPhone will Not use the full voltage available from the iPad charger, but only what is needed. The problem the other way around is that the iPod or iPhone charger cannot put out enough juice to charge the iPad as quickly as the iPad charger can.
Can the iPhone or iPod charger charge the iPad? Yes, it can, I do it all the time. It just takes longer.