A must read: Dare Obasanjo: Explaining REST to Damien Katz:

There are other practical things to be mindful of as well to ensure that your service is being a good participant in the Web ecosystem. These include using GET instead of POST when retrieving a resource and properly utilizing the caching related headers as needed (If-Modified-Since/Last-Modified, If-None-Match/ETag, Cache-Control), learning to utilize HTTP status codes correctly (i.e. errors shouldn’t return HTTP 200 OK), keeping your design stateless to enable it to scale more cheaply and so on. The increased costs, scalability concerns and complexity that developers face when they ignore these principles is captured in blog posts and articles all over the Web such as Session State is Evil and Cache SOAP services on the client side. You don’t have to look hard to find them. What most developers don’t realize is that the problems they are facing are because they aren’t keeping RESTful principles in mind.