Mark Pilgrim has posted an extensively documented demonstration of the current draft of the Atom API, a REST style web service interface for posting to and editing weblogs. It looks like it will be a fun standard to implement; the basic idea (as with all REST services) is to obtain as much leverage as possible out of the HTTP standard, using lesser known methods such as PUT and DELETE in addition to the more common GET and POST. The authentication mechanism is particularly interesting: since Apache’s support for digest authentication requires an additional module that many hosts may not provide, the Atom guys have created their own based around new Atom-Authenticate HTTP headers.
This is the only part of the current specification that I, as a PHP developer, have a problem with. To my knowledge, there is no way of directly accessing non-standard HTTP headers from within a PHP script. As such, it would be impossible for a PHP Atom API application to support Atom authentication. This is a serious barrier to adoption of the standard, especially as PHP is by far the most popular open source web scripting technology.