Ted,
At this time there is no specific version number for ArcGIS Explorer (unlike other ESRI Products) other than the newest public release being identified by the build number, hence ArcGIS Explorer 1200.
Having said that, the new ArcGIS Explorer is optimized for ArcGIS 10 yet will still work with previous versions of ArcGIS. I hope this information helps resolve some initial confusion.
Frank M.