I’ve noticed that my iPod (40GB 3G) does not really take a full charge while connected to my firewire hub at home… When I disconnect, it often has a reduced charge and generally runs for much less time (like my flight to San Fran last week) than when I leave it plugged directly in the wall where it seems to get a full charge and in fact gets to the point where it even says charged. I noticed a dramatic difference in the last trip — especially now that I am moving about with a PC without firewire and only plugging it into the wall.
I have no idea what causes the difference, but it’s real. Perhaps there’s some kind of silly safeguard on the battery though I am not sure how it would even know which way it was getting a charge…