I've had confirmation from apple europe that this is indeed normal behaviour:
"The response from our engineering dept states that the behaviout of the ipad while connected to the charger would be expected. There is measurable AC voltage across the external metal parts when an iPad charges. The measured voltage is within the SELV (Separated Extra-Low Voltage) limit, which means that the iPad is safe to touch. Additionally, the touch current is within the safety limit according to UL/IEC 60950 (Safety of Information Technology Equipment).
In the United Kingdom, Ireland, and other countries using similar AC power plugs and sockets, all AC plugs have three prongs; however, only the AC plug with cord is grounded. The shorter "duckhead" plug is not grounded"
Hope this clears it up for everyone. This is direct from apple support themselves so the source is trusted.