We first heard of Windows Phone 7 Apollo back last year as being the update Microsoft is planning to roll out after Mango and the information surfaced again this May. Turns out that there might be an intermediary update between Mango and Apollo called Tango.
According to Digitimes, Compal Commununications — the company that recently became Nokia and Acer Windows Phone ODM — has signed a licensing deal with Microsoft for using both the Windows Phone Mango platform and a new one, Tango, a platform to succeed Mango. Chances are that Apollo will be the version number bump to Windows Phone 8 and Tango will be just an intermediary bugfix/performance/feature update. Anyways, remember the name.