ESRI has posted a notice saying that there will be some updates to the ArcGIS Online services:

On August 9-10, 2007, the ArcGIS Online system will be updated to improve the network routing for service requests. We do not expect this to affect the availability of the ArcGIS Online services but you may experience some performance issues while the changes are being applied.

When completed, we expect the changes will improve the overall performance of the services. However, if you experience any ongoing service availability or performance issues as a result of this change, please post a description of them to the forum.

I see no mention of ArcGIS Explorer being affected but I assume since AGX uses ArcGIS Online, Explorer might also notice performance issues.