Do I need a step down transformer to charge my Iphone 4 in the UK? What wattage transformer is safe?
The UK iPhone charger give 5V DC at 1A (so thats 5W) on an input of 100-240V 50/60Hz with a draw of 0.25A

If you have the older two part chargers for iPod or iPhone 1st Gen, then they are fully 100-240V for all world supplies, all you need is the right connector (figure 8) to provide the AC to the charger.

If however you have the newer chargers that shipped with the 3G/3GS onwards, even though they are also 100-240V universal chargers, the pins are moulded into the chargers and you will need a plug converter.

So no you don't need a step down transformer, but you will need to provide a pin convertor.