Responsive images cook book
Andreas Bovens has published a the comprehensive round-up of use cases and example code for "responsive images" at Dev.Opera:
Dev.Opera — Responsive Images: Use Cases and Documented Code Snippets to Get You Started
You should also read Eric Portis' in-depth article about srcset & sizes. Filament Group's picturefill will be familiar, but I'll link it for the sake of completeness.
Update August 2014: here's a behind-the-scenes summary by Yoav Weiss, who's is working on the implementation: https://dev.opera.com/articles/native-responsive-images/
Update October 2015: Jake Archibald published a nicely annotated overview of usecases and appropriate techniques: https://jakearchibald.com/2015/anatomy-of-responsive-images/