During the MIX11 sessions, we came across a slide that referred to the next update of Windows Phone i.e. 'Mango' as version 7.5. At the same time, the presenter played it off as a 'mistake' even though the slides are vetted before presentation.
Now, on the Microsoft Partner Network, we can see a new section trumpeting the new version of Windows Phone which they are clearly referring to as "Windows Phone OS 7.5". (Note: latest builds of the OS pin it at version 7.1, which means MS is still a bit off from having a completed Mango build--this makes sense being six months out).
Why is this interesting? Microsoft has been in a flux over what to call Windows Phone. For the last few years, it was Windows Mobile with the version number after it. Then came the general "Windows Phone" and we were told personally that they were moving away from the version number thing. Then came "Windows Phone 7 Series", later amended by removing "Series".
Now it looks like Microsoft is going down the road of 0.5 upgrades and labeling, calling Windows Phone 7 'Mango' version 7.5, which we guess is less odd than saying 'Mango' all the time. We'll see how this pans out by the fall as we wouldn't be shocked if they change it yet again.
Update: Brandon Watson is sticking with the "no version number has officially been announced" line.
Source: Microsoft Partner Network; Thanks, @Avi_Franchise, for the heads up