Late last week, Microsoft's Windows Phone 7 Series team announced via Twitter, that from now, Windows Phone 7 Series is only called Windows Phone 7 - without Series. Also Microsoft confirmed this in a later info: "We're simplifying the name of "Windows Phone 7 Series", it'll now be just "Windows Phone 7". Back in February, when Microsoft announced Windows Phone 7 Series, the "Series" part of the product name was explained to indicate that there would be a range of phones providing choice of hardware and operator - which absolutely made sense. However, neither the end-user nor the press really picked-up the idea behind this Series-thing which only caused a lots of questions and wrong guesses.
Therefore, with feedback from carriers, end-users via Facebook & Twitter and bloggers, Microsoft decided to make things simpler by shortening the name (which makes sense because it's questionable why a product name should be be more complicated than the actual product it represents).
Cheers ~ Arne