Macs with "Charging" USB ports
Further to my earlier post on Mac models which appear to support the USB battery charging specification, a friend and I have confirmed that the August 2007 iMac and January 2008 Mac Pro also support this feature, but the June 2007 MacBook Pro and August 2007 Mac Mini do not.
That gives the following list of Mac models which will be able to charge an active iPad via USB:
iMac: "Mid 2007" (iMac7,1) and later.
MacBook: "Early 2008" (MacBook4,1) and later.
MacBook Pro: "Early 2008" (MacBookPro4,1) and later.
Mac Mini: "Early 2009" (Macmini3,1) and later.
Mac Pro: "Early 2008" (MacPro3,1) and later.
Unconfirmed but probably work:
MacBook "Late 2007" (MacBook3,1).
MacBook Air (MacBookAir1,1) original model and later.
Xserve: "Early 2008" (Xserve2,1) and later.
Definitely excluded:
MacBook Pro "Mid 2007" (MacBookPro3,1)
Mac Mini "Mid 2007" (Macmini2,1)
Probably excluded (because it was only a component update to the previous model):
MacBook Pro "Late 2007" (MacBookPro3,1)
The timeline suggests the feature was introduced with the August 2007 iMac, and added to all subsequent new models. The August 2007 Mac Mini missed out, and the Mac Mini wasn't updated again until March 2009, so it was the last to get this feature.
I also tested my iPhone 3GS connected to an Apple USB keyboard (aluminium) on an August 2007 iMac. System Profiler reported that the iPhone was being supplied 500 mA plus 400 mA extra operating current. The keyboard itself was being supplied 100 mA, as was another USB device connected to the keyboard, so in total the computer's USB port was supplying 1100 mA. This is the limit Apple documented in
http://support.apple.com/kb/HT4049
Since this article specifically mentions the late 2007 MacBook, it is reasonable to assume that model will be able to charge an active iPad.
It turns out that the Apple USB developer documentation I was referring to earlier is completely wrong in places. It claims the mid 2007 MacBook Pro and Mac Mini are able to supply extra power to the Apple USB keyboard, but I have established that this is not true, at least for my MacBook Pro. It also distinguished between earlier models that could supply extra power to the keyboard and later ones which could supply extra power to any device, but I can't see any evidence that such a distinction exists.