From what I've seen from my wife's iPhone, yes - it can charge on any USB port, even if it's not a 1A port. That being said, if you drain the battery and trying to charge it with a lower than normal charger, it will never recharge. Sort of like an engine, it's needs the correct amount of amps to start up the entire system. You can test this out yourself (for those of you who own Subaru Outbacks with a built in USB port, I can assure you, it will never recharge a completely dead battery phone).
If we were talking tablets, its much different. Some iPads / Galaxy Tabs will refuse charge on anything but the correct amount of volts and amps. I can't tell you how many times I've plugged in an iPad to its non-native charger and get the "Not Charging" sign by the battery.
This obviously excludes plugging them in straight into the computer - they have enough for charge.