According Stefania Duico, the lead of Windows Phone Italy, the Windows Phone Tango update will be officially called 7.5 Refresh. As you probably know, currently WP Mango is called just 7.5.
Adding the Refresh moniker and not changing the OS version actually makes sense, because this update major role is to lower the Windows Phone specs so less-capable devices could run it (without multi-tasking, live tiles, etc.).
The update will become available for every WP-based smartphone out there later this year. The next major release is Apollo a.k.a. Windows Phone 8. It will probably launch in Q3 this year.