Windows Phone 7.5 was introduced immediately

Users of Windows Phone 7 (WP7) which did not wait for updates on this one. 'Mango' or Windows Phone 7.5 will bring improvements 'NoDo' will be introduced first time on May 24, 2011.

Microsoft plans to conduct an exclusive preview of 'Mango' in New York and London.
This event will dissect the latest features in Windows Phone 7.5 that will be present later.

Some additional features such as Audio Bing, Bing Vision, Turn-by- Turn Navigation, SMS Dictation and so forth. Nokia, Windows Phone 7 closest associates at this time, reportedly awaiting Mango updates before the launched two first Windows mobile phone, Nokia W7 and W8.
Here are some features that will be present at 'Mango':

Multitasking and background processing.
Audio File transfer.
Transfer applications faster.
Many integrated applications.
Improvements in camera feature.