I think privacy is a valid concern when your data is stored in the 'cloud'. That is, how the data is used down the line (besides providing a backup and a sharing facility between devices) is a concern. Also, it would seem that both Amazon and Google insist on collecting a profile of their users, the latter being more extensive. Currently these are (openly) used for fairly innocuous ad targeting, but the thing is that they could be used for other purposes and/or by other parties (via sale or other contract with a 3rd party, subpoena or legislation).
Amazon knows (and has to know) about your purchases and some aspects of your ebook reading. It would seem that a major part of the motivation of routing Kindle browsing trough their servers is collecting a more or less extensive profile on a customer's browsing habits: the expense of putting the system in place and the its upkeep, including the considerable bandwidth it consumes can hardly be justified by a modest speed gain in browsing. With Amazon, a saving grace might be that you are actually a paying customer so they have to mind that; with Google, though, it is worth noting that the actual paying customers (as far as we know) are the advertisers (i.e. "You are not the customer you* are the product." *your data / profile), also Google's profile on you can be far more extensive, if you allow them to tie all searches, webpages visited, emails, your data stored in the cloud and whatnot together for profiling by being logged into a Google account.
Last edited by jolepp; 07-02-2012 at 02:43 AM.