The format of the default template containing config.xml is wrong - found in %USERPROFILE%\.cordova\wp\cordova\3.3.0\wp8\template\config.xml - there should be an XMLNS attribute in the root widget tag.
Currently it is:
It should be:
<widget xmlns = "http://www.w3.org/ns/widgets">
This is stopping users from adding the Windows Phone 8 platform, because there's a check in config_parser.js (%USERPROFILE%\AppData\Roaming\npm\node_modules\cordova\src\config_parse.js) on ln. 33 which checks that the root element is a 'widget' tag, contains an attribute and that attribute equals the w3c namespace (the third check fails).
It's a simple fix, but it's quite critical because of this.