iPhone Charging Current Zap
Sounds like a little leakage through the phones charger (i.e. capacitive coupling between windings) There is no direct short from the iPhone to ground as it is floating with respect to it (there is no circuit completed as the charger secondary is not connected to ground)

The current will be tiny (assuming there is no fault) so not dangerous, but the voltage can be quite high so it will tingle a bit. Note that really cheap imitation chargers can often be very badly designed and not meet any sort of safety standards, so if you haven't got an authentic charger I would advise grabbing one as soon as possible.

You can measure the voltage by setting the multimeter to AC, then connecting one probe to house earth and the other to one side of the charger output (either positive or negative) You can also use a scope in a similar fashion - as it is earth referenced just use the probe tip, don't connect the ground clip. 

Here's a snapshot of my scope measuring the leakage on an old Nokia charger: