It sounds like you want to generate expiring links,
http://www.ioncannon.net/s3/21/creating-s3-urls-that-expire-using-php/
You can then generate these on the fly, and supply them as redirects via your application.
So someone requests,
http://images.mydomain.com/img123.jpg
and you generate a nice, auto-expiring s3 url,
http://mydomain.s3.amazonaws.com/T154456.jpg?AWSAccessKeyId=1ESOMESPECIALIDJJAKJ6RA82&Expires=1241372284&Signature=ddfr%2BlkoSEPAL%2BGbMwlMzj6q%2BCY%3D
then you redirect the user to the auto-generated url. this way, even if the user looks at the html source they only see the
url leading back to your site, but when they click the link, they are redirected to amazon so you don't have to worry about proxy-ing
the request via your server and eating up un-necessary bandwidth.
