The video below is from a Microsoft private event for its employees which showcases some of the advancements it has made on Bing's voice search for Windows Phone.
The voice recognition accuracy on the prototype has now been improved by up to 15-percent on the back end and should have a 10 to 15 percent performance boost. Microsoft Research has also enhanced the speed at which the app can deliver results. No word on when we will actually see these improvements in an update.
Source: MSFT Kitchen