Same thing here. I tried Cloudfront, and W3 Total Cache kept giving me the ERROR: Empty Access Key. That was really strange, because it didn't have any trouble accessing my account and creating the distribution. Even tried opening a new AWS account, but still had the same issue.
So, I signed up for MaxCDN, and I get ERROR: Empty hostname. Using the latest WP 3.6 and latest W3 Total Cache.