I bought one of the fourth generation (4G) iPods - the 60GB iPod Photo - and it came with a USB charger and a USB-to-dock connector cable. I plugged it in, attached the iPod, and it charged right up.

Later, I tried to attach Jenn’s iPod (my 3G 15GB iPod) to that charger to charge it up, and it doesn’t charge - it stays on, but it doesn’t charge.

Turns out this is a known issue. The Apple Store page for the USB charger says it “supports iPod shuffle, iPod mini, iPod photo and iPod with click wheel.” Which subtly excludes 3G iPods (they don’t have a “click wheel”).

Interesting that they don’t outright say anything about this anywhere in the official Apple support section. You’d think more people would wonder about this.