Charging iPhone 4S from closed MacBook Air
I am not sure about MacBook Air, but in my MacBook Pro (Mid 2010) it will charge my iPhone if the MB is active (powered up and not sleeping) and it will also charge my iPhone when sleeping, but only if the iPhone was plugged in before the MB went to sleep.

If your MB is shut down (not active &amp; not sleeping) it will not charge your iPhone.



UPDATE

One of OS X/MacBook updates must have changed something at some point. Now, when a MacBook is plugged and sleeping and you plug in an iPhone, the MacBook will silently wake up, start charging the iPhone and fall back to sleep. It all happens with closed lid.