oh...
as I suspected...
There is the possibility (for Amazon) to limit the download of PDFs only to 3G connections...?
It will be very useful donwload mp3 (Audiobooks) or PDF (like CreativeCommons licensed books) when you are connected through your WiFi (I mean 802.11b/g)...
maybe they could add this feature with a future firmware release...?
or the only possibility is to "find an hack"...?
(I'm not so good in programming, so I ask you if it's an impossible hack to do...)