The newest official XULRunner has been released. XULRunner 18.104.22.168 matches the Firefox 3.0.6 release. For XULRunner developers, most of the changes in 22.214.171.124 are related to security fixes. You can also look at the full list of fixed bugs.
Update: We now have XULRunner builds for Windows Mobile! XULRunner runtimes now exist for Windows, Windows Mobile, OS X, Linux and Maemo (a mobile Linux). Another XULRunner port is underway: Symbian OS (S60).
Want to get started building XULRunner applications? We have an article for that.