In Userscript.org and Amazon S3, Jesse Andrews describes how he moved all of his static content from his own server over to S3. Jesse set up the virtual hosting, moved over all of the content (using S3 Fox no less), and had everything up and running inside of 15 minutes. As he notes "it took longer to write this post than it did to do the conversion."
Userscripts.org isn't some quiet site with little or no traffic. As the primary repository for Greasemonkey scripts, it is quite busy -- Jesse says that they handled 24 million requests for 307GB of static content in the last two months alone.
Jesse asked his friends to try out the new version side-by-side with the old one. In this unscientific and somewhat subjective test, the new version (using S3) was unanimously judged to be faster.
But wait, there's more!
Casey Muller wrote a really nice guide, including code for an s3commit script (to copy assets to S3) on Serving the Rails Public Directory out of S3. Casey is one of the developers of jamglue, currently in beta and serving up its content from S3.