http://www.esri.com/explorer redirects to http://resources.esri.com/arcgisexplorer/. There are now forums available, but it appears only users who are part of the pre-release can login.
update – looks like ESRI removed the redirect, but the new webpage is still up. Screen capture below in case you don’t feel like visiting.

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ESRI ArcGIS Explorer close to launching…
A new possible launch date for ESRI ArcGIS Explorer: November 14, VerySpatial has heard. Earlier, James Fee noted that the holding page for ArcGIS Explorer is up, and it’s been getting a growing number of previews of the types of……