HN user
The XDK itself is available for Windows, OS X and Linux. When you go to the download page (http://xdk-software.intel.com/) it detects the OS you are using and provides you with the appropriate download. At the bottom of the download page are links to download the XDK for a "non-detected" system.
The XDK operates identically on all three platforms, one of the reasons for using node-webkit to create the app.
Cordova 3.0 support is coming. :)
The username and password is needed to manage your cloud account; a backend server that is used for build services and to manage the wireless test and debug features. If you are not connected to the Internet when you start the XDK it will still function, but you will, obviously, not be able to use those features that require access to the cloud services to function.
You cannot use the XDK to develop node-webkit apps. The XDK itself is a node-webkit app, but you use it to develop HTML5 hybrid apps for mobile devices.
Yes, those asterisks just refer to a footnote that declares that "*Other names and brands may be claimed as the property of others." --> http://www.intel.com/content/www/us/en/legal/trademarks.html
The license you are referring to (http://appcenter.html5tools-software.intel.com/TOS/) applies to the XDK itself, not to the apps you create with the XDK. Intel is not restricting how you choose to redistribute the apps you produce with the Intel XDK, they belong to you and you can redistribute them however you wish (in source form or binary form).