Isn't the case on the iPod Touch plastic? If so, electric shock not possible. In addition, USB port is 5vdc, not enough voltage to cause a shock, you might feel a tingle if you touched your tongue to it but you won't feel it with your hands/fingers.
Before someone jumps in to say that it's the current that you need to worry about, not the voltage, let me point out that how much current flows is determined by the voltage and the resistance. I=V/R. 5vdc divided by your skin resistance (typically 100K to 1M ohms, depending on whether your skin is wet) means less than .05 milliamps of current flow, not enough for you to feel.
My guess is you got some sort of a static shock.