Trying to understand S3...How do you limit access to a file you upload to S3?  For example, from a web application, each user has files they can upload, but how do you limit access so only that user has access to that file?  It seems like the query string authentication requires an expiration date and that won't work for me, is there another way to do this?
Have the user hit your server
Have the server set up a query-string authentication with a short expiration (minutes, hours?)
Have your server redirect to #2
Amazon S3 permissions