The web service companion for the Xml Rules Engine has been released.
Although still in beta version... it provides a sneak preview to what is to come with the downloadable version. Another point of interest is that when the XRE IDE is released, it will have native support for the service, so when you're working on any rules, you can always parse them through the latest version of the engine.
This can help you in deciding whether an upgrade is nescessary or not.
Web service information can be viewed here: