The Kindle apps I have installed on my Blackberry and netbook allow me to pull up any book I've purchased through Amazon as an "archived" item and download it to both platforms. I think there's a limit to how many devices I can have registered on my Amazon account (though I also think it allows you to un-register an old device you're replacing), and I make a special point of downloading them all to my netbook to keep because I don't want to trust entirely in Amazon to make them all just as available for insta-downloading three or five years from now as they are today. Also, if I buy a Kindle book not through Amazon (e.g., I've most recently being doing some buying directly from Baen Books) I'm going to have to either seek out a download link via my webbrowser or physically copy the file over.

Mind you, whenever I open the app, there's a second or two where the program wants to connect to Amazon.com (to verify registration, update things like bookmarks and last-page-read), and I haven't yet made a special point of opening the app when I'm somewhere without a decent mobile internet connection. So I don't know if the outcome if I try pulling up an e-book at my father's is going to be that next time I try opening that book it won't have advanced by the amount I read in the offline session -- or if it saves the changes and updates them the next time I open it where I've got a connection -- or if it just won't work at all.