I installed the WP8 SDK, and try to add the WP7 platform to the HelloWorld project, and get error because of .NET Framework isn't good, so I can found a fix here : http://stackoverflow.com/questions/19812125/cordova-platform-add-wp7-fails-msbuild-error
by modify the check_reqs.js file.
But after fix this, when I try to add WP7 platform I get this error :
Checking wp7 requirements...
Creating wp7 project...
TypeError: Object #<Object> has no method 'config.xml'
Use of -d option show this more :
I try to use the 3.1.0 version and it work fine.