OverTheAir: Progressive Enhancements, Daniel Bodart
What is it? See wikipedia article. Not the same as graceful degradation: feature support not browser detection. Similar to TDD in terms of motivation and rewards: start with a dead basic service and add features which are made available where possible.
Text -> HTML -> XHTML -> XHTML Basic -> XHTML Mobile
The act of developing using this technique encourages better markup and accessibility.
Agile-ish in that you do the core stuff earlier, bells and whistles last. Early iterations deliver the most value. Sounds sensible.
Demo of E4 site, using the "behaviour" library. It's important to ensure that if your progressive enhancement fails, it fails completely without modifying the document messing it up.
Behaviour is to have the client make decisions, not the server. This helps deal with future devices which might come along without having to maintain a large database of pages. But it does mean that you're relying on lowest-common-denominator and the client being able to handle one basic version of your service.