All the hard work I've put into DoneCal pre-optimization has paid off: Got a Grade A with 92 percent on YSlow!
It's worth noting that Amazon Cloudfront supports custom origin servers now, so you can have both the varying Gzip encoding and the benefits of distributed servers at the same time.
Doesn't that mean that I can "back up" by CDN with my Nginx. As a fallback that is. How does that change how CloudFront can serve negotiated gzipping?
Origin server means the place that CloudFront gets original content from, and that Accept: headers are sent over HTTP 1.0 to Nginx. See here for more information: http://www.nomitor.com/blog/2010/11/10/gzip-support-for-amazon-web-services-cloudfront/