Since i don't carry the charger around with me, I discovered my iPod mini can be charged by simply connecting it to a powered USB hub (and it will charge as if it was connected to a computer or a charger). It takes more time, but it works.
[robg adds: Or you could use your AirPort Express, or perhaps (at least according to the comments) via USB on (some) sleeping Macs.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070126100345550