On the train ride to and from work today I read RESTful Rails Development.  I've already done a fair bit of reading on REST and I've worked with several RESTful APIs lately (Amazon, Flickr), however, I found the RESTful Rails Development article a good introduction to what is at the heart of REST API's.  Namely, the idea of resources, and more specifically URI addressable resources.  The support for REST within Rails 1.2 adds a lot of nice features, and looks like a great way to support multiple output formats (html, xml, javascript, rss/atom, etc.) for applications that are oriented around resources.  Anyway, if your interested in REST or Rails, it's worth a read.