While it seems Amazon doesn't offer any type of real hotlink protection, from doing a bit of research there is the ability to at least limit hotlinking using "query string rest authentication" The info is here at the bottom http://docs.amazonwebservices.com/AmazonS3/2006-03-01/index.html?RESTAuthentication.html
It's pretty much spanish to me, but it seems that it allows you to create a "pre-sgned" request then set it with a time limit. Basically the image would be served then the image url would be invalid after X time. Apparently this method is similar to hotlink protection.
It would be a nice addition if possible.