I was trying to transfer some files to my Kindle using the USB port. I have a Kindle 2, and a Rocketfish Micro USB cord. The Kindle does charge up so it is drawing power through the cord. But the Kindle does not show up as a device attached to my MacBook. My external HD (Firewire) and iPods (USB) all work perfectly through this USB hub. I did call Apple (using my 30 day warranty for installing Snow Leopard) and the tech support person did not even know what a Kindle was. Amazon says it's an Apple problem. Apple doesn't seem to have any clue. I've seen from various threads that USB devices do seem to have intermittent problems. Is the Kindle one such device?
Some devices do not work properly through a hub. Plug the kindle directly into your Mac and see if it works. Just because some devices work through a hub, doesn't mean all devices work through a hub. Apple does not support third party devices or third party hubs.
You said in your post that all your USB devices are plugged in through a USB Hub, not directly into your Mac's USB port. Remove all your USB devices, including your USB Hub, and plug the Kindle directly into the Mac's USB port.
I found the answer to my own question. The cable I bought (at a Best Buy made by Rocketfish) only works as a charger and doesn't transfer data. I have no idea why a company would do such a thing. But it has.