While we are on the topic of CDNs I'd like to remind everyone:

S3 is not a CDN, and you should not be serving assets from it directly

S3 is convenient, but assets are only served from a single region, and sometimes traffic from other geographic regions can be very slow. I'm in Europe and personally have seen excruciatingly slow transfers from us-* S3 buckets (10s of kbps, over my 1gbit connection). Even if you only expect files to be requested without being edge-cached, just sticking Cloudfront on top can delivery files much faster.