"Metro" is Microsoft's Code name for the user interface that they debuted with the Zune HD back in 2009 and has been a major part of their UI design language in Windows Phone as well as the upcoming Windows 8 desktop operating system.

Well, the UI isn't going anywhere but the name is. Microsoft has stopped using the Metro name for the UI effective immediately and has also asked the developers to stop using it in their apps. The reason for this is due to legal issues with a German company over the name "Metro".