SonyEricsson's standard USB datacable doesn't charge the phone, altough the USB-port could be able to do this. The reason the cable is built this way is unknown, one good guess is that the cable should work the same way wheater it's connected to an unpowered USB-hub or a USB-port on the computer.
So, is it possible to convert the datacable so it can charge the phone? Let's see..' My research begun with disassembly of a car charger, to see what's inside. The first thing I notice is a mc34063 ic, which turns out to be a DC-DC-converter. Not a surprise, the cable is powered by 12V and gives something like 5,1V at 450mA. A look at the PDF for mc34063 and at the carcharger circuit gives that this is simply a step-down converter, with an additional 1N4001 diode on the output (to lower the output voltage?).
So, it should not be any problem charging with the USB-cables 5V. Let's disassemle the datacable! Damn the modern electronics, almost impossible to open them without destroy them. But this one was pretty simple, the to halves are glued together so a small screwdriver and some violence got it open. Be careful though, the screwdriver can leave the circuit a little much to disassembled. Which was justed what happened to me, therefore I haven't investigated this further.