There's been a lot of speculation on when exactly Windows Phone "Mango" will launch--or even what that means exactly. The reason it is a questions is because there are device upgrades, there are device launches and there's no reason to think the two are tied together. Hence we've seen September 1st, August 25th and the vague "this fall" all thrown around and in fact, they're most likely all true for specific situations.
Microsoft's marketing chief for Scandinavia, Peter Wissinger has taken to Facebook to clarify the "Mango" launch. In short he tells WinMobile.se that it is completely up to the OEMs to decide the timeline for launch of the next-generation of Mango devices. In fact, as has been speculated, Microsoft came out ahead of schedule this time with "Mango' which is why OEMs, who just received the RTM version, may seem a little behind--perhaps they weren't prepared to have it all ready this early (although Fujitsu seems prepared).
Of course in the US, OEMs still have the FCC and carriers to deal with before launching a device, making the situation more complicated. We're expecting a similar style to last years launch: announcement in October, availability (on AT&T, at least) a few weeks later with numerous devices.