Just a small information up front: I tried the following modification myself and it worked for me. When you decide to do this modification also, you can loose warranty or even break your devices. Which means: Everyone who is doing this modification is solely responsible for the consequences!!
Recently I got an iPod nano 2nd generation for free. I like it very much. The only thing I do not like: Charging is only possible through the data cable and a running computer. But I didn't want to spend 40,- € for an original Apple power supply.
Therefore I bought a cheap USB power supply set through ebay, which contained of a power supply for home, one for the car , and a data cable. I favoured USB power supplies as there are lateron usable for otehr devices, too. In this case for my mobile phone and an external image tank.
Those cheap power supplies only have on problem: The iPod nano is not being charged. After some searchign I found a reason for this which lead me to this modification as described here. Generally speaking this modification can also be done using a short USB cable typa A-A male/female which you cut into two pieces.
The reason for the power supplies not charging was simply that only the both outer pins of the USB connector are used (GND and +5V). Both middle data pins were not connected and therefore the signal voltage levels were not defined. It looks like the iPod is doing some kind of checking. (I guess that this checking is done by the charging circuit, because this behaviour can be found with various other devices, too.) To allow charging the data pins have simply to be set to a defined voltage level. The folowing circuit is totally sufficient:
As you can see two simple voltage dividers are enough to set the data pins to a defined (middle) voltage level. Resistor value can be 22kΩ od 47kΩ. Basically the resistor value is not that important. It only should not get too small, because otherwise there is too much load on the power supply. By using the power divider you also make sure that there is no short and the drivers of the Ipod are not damaged.
I did this modification in the powersupplies itself. Of course you then have to be very careful to not create a short and isolate everything properly. Make also sure that no part of the circuit extends to teh 230V parts of the power supply. (Otherwise the iPod might get damaged!)
Small hint when integrating the above circuit into a cable: The power wires are usually a bit thicker than the other ones.
BTW: Such a changed power supply can be also used with an iPhone 4!