@ixtab: the cover/thumbnails thingy is probably not pretty... AFAICT, as soon as there's an ASIN specified (exth 113), it will try to download the thumbnail from Amazon via HTTP for this ASIN (it's visible in the logs, might be run as part of the indexing/catalog process).
The workaround Calibre implements is sending said thumbnail at the same time as the book. The FW sees the thumb, doesn't try to re-dl it. It obviously doesn't implement this for the cover view, only the thumbnails
(And the 'enable share' thingy just prevents Calibre from injecting something in exth 113 if it's not already set. On the other hand, this field is *always* set for books purchased on Amazon. I'd be curious to know if those end up having a proper cover, wireless off or on...).
I haven't looked at the difference in thumbnails vs. coverview handling, but I'm guessing it's in the same spirit... (Do they end up cached somewhere in /mnt/us/system, like the thumbnails?)