Microsoft has official changed their name of the new OS to Windows Phone 7 instead of Windows Phone 7 Series.  I think it's a great decision because Windows Phone 7 Series is a little hard to remember.  Dropping the "Series" makes it really easy to reganize and remembered.

Here's statement from Microsoft Windows Phone's twitter account.

Tis the season for Series finales. We've got one too - dropping the ‘Series' and keeping the 'Windows Phone 7.' Done.