Amazon has also provided a Ruby API for S3 at http://amazon.rubyforge.org
It won't be long before someone writes a mod to file_column or other similar Rails plugins that can store to S3 instead of the local harddrive.
A couple months ago, I had the chance to talk to someone who was creating a Rails application involving serving up images. He was writing up a proposal for some investors he is talking to. He needed something that would scale with volume. The solution at the time was to hash the filenames of the photos and distribute them across multiple servers. Of course, had I known about S3, the solution would be simpler: on uploads, dump it into an S3 bucket, then call the "url_for" method to get back an url and store that into the local database.
For more information: http://www.amazon.com/gp/browse.html?no