Exactly,
The certification labels from Apple.
iPhone charger 5V at 1000mA
iPad charger 5.1V at 2100 mA
iPad w retinal 5.2V at 2400mA
The reason why some of you have had problems charging and some have not, depends on which charger you have.
If you are using the 10W charger, the 2100mA is not enough to charge the iPad if it is in use.
If you are using the 12W charger, the 2400mA is enough to charge while the iPad is in use.