What is the technology behind Dropbox?

Does anybody know how Dropbox manages to give 2 GB free to each user? Yes, the paid users pay for it, but there must be something else going on? Do they do some form of compression of data (sounds like a bad idea for speed)? There are these guys at Bitcasa (http://www.bitcasa.com/about/) who are planning to give infinite storage to each user for $10 per month, arguing that when you consider many users, each user will only have about 25GB of unique data (http://techcrunch.com/2011/09/12/with-bitcasa-the-entire-cloud-is-your-hard-drive-for-only-10-per-month/). So maybe Dropbox does something like this already? Speculate.