Your point is well taken. The little Apple cube shaped chargers are 5 volts dc and are rated at 1 amp. Those Motorola chargers with the mini-usb plugs are 5 vdc and are typically rated at 550 milli-amps (mA), or 0.55 amps.
I've got an electronics salvage store close by and I've played with plenty of different chargers with usb outputs. So far, as long as they meet the 5vdc and a minimum of 550 mA my 3Gs will allow charging. But anything less than the 550 mA and it won't go into charge mode.
I'm an electrical engineer by training but I have to admit that I know little about battery technology. Based on what little research I've done, batterys are not damaged by low charge currents but can be damaged by too much charging current.
I'd love to hear from some battery experts on this.