In case you're wondering why a properly functioning charger gets warm... It's because, like all chargers, it's less than 100% efficient at supplying power. You can read a little about efficiency of AC adapters here:
AC adapter - Wikipedia, the free encyclopedia
I'm not an electrical engineer but, as I understand it, the efficiency of a power supply basically tells you what percentage of the input power is put to good use. The remaining power is wasted as [usually undesirable] heat output. Given that phones are designed to charge as quickly as possible (without damaging the battery too much), they can suck down a lot of power in a short time. This means that even if the charger is relatively efficient (80-90%?) the 10-20% that becomes waste heat adds up to something you can easily feel in your hand.