The familiar 32-pin connector on the iPad gives a sense of security and continuity. I had hoped that all my various USB chargers would work and provide as much power as necessary for simultaneous use and charging. Sadly, this isn't the case. The iPad comes with yet another charger, this time of 10watts, to add to the impressive collection of Apple PSUs–45w for the Air, 60w for the MacBook and 85w for the 15in and 17in models. I have a drawerful and I'm not happy at having to have yet another unit to choose from.
I have often wondered whether, for instance, I would fry my MacBook Air if I plugged in the 85w charger from my MacBook Pro; or would the Air just take what it needs? And would a 45w Air charger keep a MacBook Pro alive in emergency and, maybe, offer a charge if the computer is sleeping? Apple, of course, will not make any statements on such issues, but I have often been tempted to try. Tempted is the word, of course.
No doubt, though, an iPad owner will be more than tempted to plug his iPhone into the 10w iPad unit. If not tempted, he'll probably do it in error. Apple must have thought about this and, presumably, aren't all that worried.
MacWorld have posted a comprehensive review of iPad charging options.