I know this is no complete answer, but AFAIK they use binary diffs to detect changes in the client and a dæmon that monitors the dropbox folder. I'd also assume that the server has binary de-duplication. In their YC application they explain some other stuff: http://files.dropbox.com/u/2/app.html
Thanks for the reply , i was more interested in the technical
aspects of the client / server for example
i know they are using python , but how did they compile it for windows ? py2exe ? , and how about server side ?