Windows Phone Tango is a minor iteration to the current Windows Phone 7.5 and adds compatibility to the platform to support lower end devices. So it wasnt a big surprise that Microsoft chose to officially name the new version Windows Phone 7.5 Refresh instead of giving it a separate build name.
The news comes from Stefania Duico from Windows Phone Italy. This also reduces the focus on an interim build as Microsoft works it's way towards the next big version which will be Windows Phone Apollo (a.k.a. Windows Phone 8)
The Tango update for Windows Phone is expected to be available for the entire lineup of Windows Phone devices. While it is mainly aimed at bringing support for devices with 256MB of RAM it also has some new features and bug fixes. Low end devices will have a slightly lesser experience as certain features like multi tasking and live tiles will be disabled.
source - Windows Phone Italy