Strange that nobody mentioned this yet: out of my cloud books I tried to download to my device, more than half could not be downloaded via 3G and I had to use Wifi for that. I am not sure what the file size limit is, but it's too bad they can't make an exception when it is their own books you are trying to download. I understand personal documents having to be limited, or browsing, but books should all be downloadable via 3G, or at least you should get a warning that if you purchase a specific book, you will need Wifi to download it.
My books are all rather hefty, with many of them having a map in the beginning of them (SciFi, Fantasy and Utopian).
I was upgraded to 3G because of my screen issues, so it's just too bad for me, but if I had paid extra for it, I would probably be livid about it.