The newest official XULRunner has been released. XULRunner 18.104.22.168 matches the Firefox 3.0.4 release. For XULRunner developers, most of the changes in 22.214.171.124 are related to security fixes.
For those interested in the mobile versions of XULRunner, nightly builds of XULRunner for Maemo can be found here. The mobile tinderbox is also up and running.
Want to get started building XULRunner applications? We have an article for that.