Ken Shirriff has torn down an iPhone USB charger, and the results are impressive:
Apple’s power adapter is clearly a high-quality power supply designed to produce carefully filtered power. Apple has obviously gone to extra effort to reduce EMI interference, probably to keep the charger from interfering with the touchscreen. When I opened the charger up, I expected to find a standard design, but I’ve compared the charger to the Samsung charger and several other high-quality industry designs, and Apple goes beyond these designs in several ways.