@kettchhttp://www.zdnet.com/blog/microsoft/microsoft-gives-windows-phone-developers-a-refreshed-non-rtm-mango-build/10201

"Although we will be marketing the 'Mango' release as 'Windows Phone 7.5′, the 7.5 phones (will be running) 'Windows Phone OS 7.1′. While not an ideal pairing, it's similar to how Windows 7 is actually Windows OS version 6.1."

so 7.5 is the version for consumers/marketing and 7.1 is the version for developers/technical ?

If its really like 'Windows 7' and 'NT 6.1', even developers/MSDN docs should refer to it as 'Windows 7', while '6.1' is only used in code when you want to check the version, but 'Windows Phone 7.1' is in the SDK and docs, huh.

even ms guys use them interchangably. do you use 'Windows 6.1' so often ?

http://blogs.msdn.com/search/searchresults.aspx?q=phone+7.1
http://blogs.msdn.com/search/searchresults.aspx?q=phone+7.5

and Wikipedia is confused http://en.wikipedia.org/wiki/Talk:Windows_Phone_7.5