Having recently spent the best part of a day modifying the phpBB codebase (3.0.7) to allow me to serve static files via a CDN I can say that I am greatly looking forward to hooks in 3.1.
To save you all a little effort I’m gonna share the method I used below; I’m not going to go into great detail about template and theme issues, as those are a fairly straightforward matter of swapping the old local URL’s in your template/theme for the new public CDN URL’s and clearing the cache. But rather I will explain how to modify the core files to serve smilies, forum ranks, forum images and your image set which is a tad more involved.
I’ve recently had cause to start using S3 on a personal project, and while for the most part I couldn’t be happier, there is one thing that is irritated more than I thought it would. Namely, you can’t batch edit meta-data for files from within the native AWS console.