So far there is no mobile phone released that is using Windows Phone 7, but developers if you want to develop Windows Phone 7 applications, then you have to use Silverlight or XNA.

Mobile phones running on Windows Mobile 7 will be shipped from this year holiday season. Windows Phone 7 is not just a name of change from Windows Mobile, its a total new thing, a complete and unique new mobile phone OS.

You may be interested to read this post Windows Phone 7 Unveiled At WMC 2010