ArcGIS Online Map Viewer accessed through ArcGIS.com automatically determines the field type based on the value of the cells. So your field is being interpreted as a date field. Currently, there is no way to change this type. This will be coming later.
However, ArcGIS Explorer Online has this ability now. So if you create your map in ArcGIS Explorer Online, you can set the field type. Click the Add Content button then choose Import. Then save your map and it can now be read properly by either client application.
Hope this helps,
Mike
