The ArcGIS Explorer Blog just announced that AGX build 440 is available:
The ArcGIS Explorer Team is pleased to announce that today, at approximately 1:58 p.m. PST, the newest version of ArcGIS Explorer - Explorer 440 - was released.
If the ESRI servers are your home servers, you’ll be notified that there is a new version available the next time you start the application. Just follow the instructions to download and install this new release.
The “What’s New in ArcGIS Explorer 440” page has a list of all the enhancements (I think many will appreciate the ArcIMS improvements since that is the biggest complain I always hear).