For example, say Terracotta has a S3 repo at http://download.terracotta.org. If you click on that link, you'll get an XML file. Now I want to be able to see what we have under http://download.terracotta.org/maven2, you'll get nothing but an error trying to go there.
Maybe it could be of use for you.
Note: S3 doesn't serve index.html file automatically (after all, it's not a web server) so you have to hit the index.html explicitly.