There was an article a few months ago about a little company who figured out how to make their battery-powered emergency USB charger work on iPods. And then again on newer iPods. And then again on even newer iPods.

Turns out Apple are playing somewhat with the USB electrical current rules and use specific resisters across the datalines in their charging things to tell the iDevice when it can take its (usually) motza charging current. That's why it won't charge on a laptop: it's made so it can't.

Wade.