UPDATE: True with iPad 2 also. Fix: charge your iPad fast with a better cable and charge your iPad faster from computer USB ports
The iPad needs more current to charge. Not a big surprise. What is surprising is that Apple did not include ( or accessorize ) a USB power injector that allows you to sync and charge at the same time. The iPad will charge slowly off USB if the screen is off and seems to stay about even if the screen is on and hooked to a computer USB.
It also appears that some aftermarket or longer cables will not charge the iPad even if plugged into the included 10 watt power cube. I assume this is due to more resistance on longer cables or cheaper or inferior wires on the aftermarket cables. What is very strange is that it seems to happen on older geniune Apple cables also. The Apple Store only sells one 'universal' cable, so perhaps the current one is better.
The dual ended USB cables commonly used to power portable USB hard drives do not seem to help much with iPad charging. This is probably due to negotiations and safety built into the iPad so that it doesn't over draw from the USB ports and burn them out.
Finally, the slow sync and backup issues with the iPad and different cables. There may be something to this. According to the USB specifications, different data rates are allowed at different charge rates. This could be to avoid inductive noise when more power is transiting the cables or it could be to not tax circuits that are being forced to provide current and transmit data. It seems that a longer or inferior wire would lower the line voltage due to resistance and thus increase the amperage drawn. This may be the reason the data rate drops. Or not.
I can say that I have seen the battery drop when plugged into the 10 watt brick while using it with some cables. I switched cables and it began charging. Both times it showed to be charging. Disassembly of an Apple cable shows very small gauge stranded wires. Thinking of trying to build a custom cable with more robust 5V and ground wires, just to see.
