Tipsy,
How portable do you mean? 240V, 12V or it's own battery.
I have a 6600maH battery with 2 USB charging ports one 1A and one 2.1A (this is the important spec for iPad charging). I got it from DealExtreme (import from HK) This sort of thing is now avail at Dick Smith ..I think Belkin brand ..but dearer.
Mine gives a full recharge of the iPad and has the added benefit of recharging phone/Dual Bluetooth GPS if they get low.