The Mango update to WP7, with its 500+ new features, is now supposed to land on September 1st, which means that we'll probably see the new handsets that will be rocking it in greater detail then. Apple's iOS 5 update will most likely be available in September too, and Google's Android Ice Cream Sandwich is allegedly pulled to October, in order to fight the competition sooner than planned.
If we indeed see Windows Phone Mango get commercial on September 1, this will coincide with the biggest European electronics expo, IFA 2011 in Berlin, where a lot of phone manufacturers might be showing WP gear. Considering that Acer, ZTE, Huawei and others joined the bandwagon this year, they might introduce their first handsets with Microsoft's fresh mobile OS in three weeks and change, between September 2-7, when IFA 2011 will take place.
Moreover, Windows Phone is selling way better in Europe than in the US, so it will make sense to show off the new gear at a European trade show first.