Baked is probably right; but it also fits with Apple's general philosophy. They really seem to hate doing low end stuff that might tarnish their reputation. They could easily have said "works with pretty much any USB port since the invention of dirt" and they would have been correct(pretty much every other USB connect MP3 player does this); but they might have run into a situation somewhat like the following
Person1: "I just got the new Ipod and the transfer speed is t37 suxx0r. I meet all the specs and everything, WTF?"(person one has some godawful usb1.0 contraption that seemed rather nice in 1996)
Person2: "Hehe, n00b, my cheap illuminated-happy-dragonfly-quality-brand player has l33t transfer speed."(person two has proper usb2.o and a modern bugless motherboard)
Persons 1&2 "Apple suxx0rs!"
It is much better for them to say that it only works with USB2, so that anyone who doesn't know what they are doing gets a nice fuzzy fast experience, and those who do can make it work; but also know why it isn't so nice.
Something like the hypothetical Powerbook G5, really. If Apple was like the x86 world we would have had the G5 powerbook ages ago(any engineer who can put a P4EE into a laptop, which has been done for years now, would laugh at the challenge of putting a G5 into one). This would, though, mean that there would be a bunch of big, loud, heavy, very hot running Powerbooks out there. Then people would start associating Powerbooks with hernias and leg burns, which would be really bad marketing. Instead, Apple denies the top end what they would like until they can get it working as they want it to.
