TellMe used to be a separate company specializing in voice recognition technology, until Microsoft swooped in, bought and merged them with their own exiting voice research team in early 2009. Flash forward a few months and we finally saw the first iterations of TellMe on the Samsung Intrepid in October 2009, giving us an idea of what to expect from the future partnership.
Mango will be improving the user experience with the option to use speech for text input with SMS conversation. More control will be at hand with replying, having messages read out loud and more (see our video demo with Bing services here).
But the big question is: Will Windows Phone developers ever have access to these tools? As of now, the answer is 'no' but Microsoft is starting to budge on the issue and they look like they will be opening up their TellMe voice services for WP7 developers. No timeline is given, but they are now registering developer interest via a list. So to all devs: go voice your opinion on the matter and hopefully Microsoft will movea a bit faster.
Head here, near the bottom, to register your interest: http://www.microsoft.com/en-us/Tellme/developers/