Connecting LED to iPhone's dock connector
Looking at the pinout there appears to be 3.3V output available at pin 18, so for a typical 5mm red LED of Vf 1.8V, with 20mA through it:

(3.3 - 1.8) / 0.02 = 75\$\Omega \$

So 75 ohm from pin 18 to LED anode, LED cathode to one of the ground pins (e.g. pin 15)