According to Digitimes who has one of the worst track record when it comes to “sources” and rumors, the release after Mango will be called Tango. We already know that the next big release which is said to be Windows Phone 8 and is speculated to be running the ARM Windows 8 Kernel is called Apollo so if this Windows Phone Tango is real it will most probably be a minor update.
Compal has signed with Microsoft for licensed use of the Mango platform and Tango, a platform to succeed Mango, the sources noted.
Then again, remember that most of the time Digitimes + sources = fail. GRab some salt.