On 12/05/2011 09:04, Ret. wrote:

[...] When my wife's iPod is on charge (using a non-Apple compatible
charger), if you pick it up and run your finger lightly over the
metal surface, there is also this 'buzz' sensation. I wouldn't
describe it as an electric shock - just a very very mild 'tingling'
sensation.

These 'tingles' are usually the result of capacitive leakage between the
mains and the case of a Class 2 ('double insulated') appliance. The
relevant safety standards usually allow up to to 0.5 mA leakage at 50 Hz
(more at higher frequencies). It's completely harmless[*] but can be
annoying and/or worrying. (The 'return' path for the current is via the
body's natural capacitance to earth, typically a few hundred
picofarads.) It's becoming more common with the move away from
iron-cored transformers to switch-mode circuits in wall-wart PSUs - a
consequence of the WEEE and EuP directives.

With a high impedance voltmeter (any DMM these days) you'll typically
measure 100 volts or more between the case and earth. The source
impedance of these errant volts is very high though, and just touching
the case will lower the touch voltage considerably. Temporarily earth
the case via (say) a 10k ohm resistor and measure the voltage across the
resistor if you want to get an idea of the available touch current.

As the output of the charger is low, I am assuming that there is no
danger from this...

Unless there is a fault, no.
[*] One case to be aware of is where several items of Class 2 AV
equipment are interconnected by the signal wiring. Here the available
leakage touch current will add up and can reach dangerous levels (3.5
mA is considered dangerous, 10-15 mA can be lethal). Earthing the
system (either directly or via a resistor) is advised in such cases.

--
Andy