If you remember the original iPhone came with a mini brick, same style as iPad charger. Will this charger charge the iPad. Also, as long as the iPad says it charging, it is charging, but how do you know if it is go at a full charge speed.
However, I'm very unhappy with myself when i use my iPod charger (I'm guessing it is the same as your iPhone one). I have used it 3-4 times and can say that the iPad does not hold the charge as well. It makes no sense if it is showing 100% it should be the same as 100% on the iPad charger, but it isn't at all.
I can go 2-3 days without charging normally, with the iPod brick i was lucky to make it thru one day. Now, something could be wrong with mine (except my touch holds the charge with it), but I've sworn it off.
If you examine the microscopic writing on both bricks, you'll find that the iPod charger is rated at 5 watts, while the iPad charger is rated at 10 watts. While the iPod charger will eventually charge the iPad, it will take longer and doesn't provide enough charge to power the screen while charging the device. It just isn't particularly useful, though my understanding is that it shouldn't harm anything.
I labeled our iPad-specific chargers when I brought them home, both at the brick and at the docking end of the cable. We have four of the original chargers in the house already, so this way when I grab at a cable in any given spot, I know at a glance whether or not I have the right one!