Well, it's either power supply or extra batteries, because the battery in the iPod is a proprietary lithium unit.

Or actually, now that I glance at the Apple Store site, it's not extra batteries per se, but one battery pack device (made by Belkin but nicely customized for iPod) that then takes AA batteries. So it's either that or the power supply. The Apple power supply is nicely compact and light, actually.

There is one other option: if you have a laptop with a built-in *6-pin* FireWire (1394), which is the sort that provides power through the FireWire cable, you can charge the iPod from that. Apple's laptops provide this sort of FireWire connection, but my recollection is most others do not and most/all add-on PC Cards also do not. This is of course also only a reasonable option if your carting the laptop anyway.