Apple hasn't slept on the spot when it comes to batteries: the iPad, for example, demands so much power that no USB port can handle recharging it at a viable speed and even those capable of doing so take much longer than the wall charger. Now that the iPhone 4 has a bigger, better and more efficient power source, does the same apply?
This is exactly what When Will Apple? I wanted to test it with an experiment capable of killing anyone of boredom: putting Ma's new smartphone to do a full recharge using the two possible means, a USB port on a MacBook unibody (launched at the end of 2008) and a 120V / 60Hz socket and take a load reading every five minutes, for more than two and a half hours.
The result is somewhat obvious, but at the same time comforting. The iPhone 4 recharges faster when plugged in, but the difference is not absurdly large, taking only half an hour less (about 23% less; that's half the difference on an iPhone 3G (S)). However, keep in mind that the time it takes for an iPhone 4 with a completely depleted battery to be used can be used considerably longer when connected to USB: 4min49, almost two minutes longer than when connected to a wall outlet.
What does that mean? If you use your computer to recharge your iPhone 4, the difference will not be to big. However, if his battery runs out and you have to make / receive an urgent call or leave in a hurry, the plug may be the best alternative.