Today Microsoft has renamed and dropped “Series” from upcoming Windows Mobile platform called Windows Phone 7 Series. Now it will be called as “Windows Phone 7”.
Windows Phone 7 sounds better than Windows Phone 7 Series, they may want to get a synchronized numbering scheme with the Windows ecosystem. So it will make a lot more sense to drop "Series".
However Microsoft didn’t reveal any reason, why they renamed it and the announcement was made over official Windows Phone twitter page.