Hmmm, I know CDNTools supposedly does this, but dono whether it works with Multisite... you might check with the author. * scratch that, just saw that S3 is "coming soon" - now it's ust CloudFiles (Rackspace)
We also run W3TotalCache and it MAY auto sync media, though I'm not sure if it does. http://wordpress.org/extend/plugins/w3-total-cache/
we have all our static content running on Rackspace Cloudfiles and are very happy with it; it's also less expensive than Amazon. but it's not set up to auto-sync as we don't have the need for user-shared images, etc, for us it's just the site images that don't really change much.
Have an awesome day!