A test report out on the ways to charge an iPad. Using anything like an iPhone charger or USB port is going to take forever, but the time from 0-80% charge is decent for the factory plug-in charger at 3 hours. Interestingly, the Griffin PowerBlock for iPad is even faster at 2.5 hours, making it a good choice for a auxiliary charger.
A couple of notes they had: Make sure you get the PowerBlock that says iPad rated. The same name is used for their iPhone models. Also, once a month, Apple recommends running the battery down to 0 charge and then completely recharging the iPad.