Surely there's a risk the iPad charger could fry the iPhone? The iPad charger is higher power than an iPhone charger. I'm no electrician... but if the iPhone is designed to be charged by a low power charger and you plug a higher power iPad charger into it then could the iPhone be damaged? A friend of mine used an iPad charger to juice up her iPhone and she said the iPhone got quite warm while it was being charged which doesn't happen when charging with the iPhone charger. To be honest I dont really know the answer, but personally I wouldn't risk my iPhone by charging it up with an iPad charger... unless someone suitably qualified told me it's absolutely safe to do so.
Apple's iPad charger is listed on their website as compatible with iPod touch, iPhone and iPad