I tried out the new functionality in ArcGIS Explorer Online by dragging and dropping a CSV file and zipped shapefile into my map (very cool!). I then played around with pop-ups and symbolization and saved my map. Now, when I try to open my map, I get the message you see, attached.
The map could not be opened. The document is invalid or corrupt.
Sorry, this should be in the ArcGIS Explorer Online post, but I cannot move it there.