That argument gets fully charged and overheated ;-)
Further on, it's pointed out that the chargers are just a DC power source and the regulation part is in the device itself so it won't overcharge.
I got lost half way through the argument but for what it's worth, I use the same charger to charge an iPad 1 and an iPhone 4s. You can tell the iPad has been going a while(!) and the phone was bought soon after release (whenever that was) and both batteries seem fine.
I went through this thought process a while ago too but I always worry a little about the quality of such online pronouncements. However, the clincher for me is that Apple describe replacement iPad chargers as being suitable for iPhones too (see this link to the Apple online store.
Douglas Cheney 