Gizmodo has done a simple verification on a few iPad charging solutions and come up with the above illustration. They put, bundled iPad charger, Griffin power block, iPhone wall charger and an uSB port off a new Mac Book pro. All these chargers are capable of dynamically adjusting to the power demand and are obviously interchangeable.
The best choice seem to be Griffin Power Block, which took 2.5 hours to bring iPad juice mark to 80% from 0. If you are going to get one of those. make sure it says "iPad" on the package. Also do not forget to follow Apple's recommendations for optimal iPad battery lifespan.
Gizmodo has more info on juicing up an iPad.