The ability to play your music via voice command was something that Windows Mobile users were accustomed to back in 2006. To not have the ability to say "Play <insert artist>" on a Windows Phone in 2012 is a bit absurd but we'll assume that something had to be cut for those 500 new features in Mango.
However, an app is now available that fulfills this task: Speak To Play by developer Puno-Z. The app is a clean, minimalist design with a large microphone graphic that you "push to speak". From there you can tell it to play an artist, song, album, playlist or even genre.
In our usage with it tonight, we'll agree with those multiple five-star reviews in the Marketplace--it works very well. We had no issues with it recognizing what we wanted to do and it gets the job done. It's of course still not as nice as being able to run TellMe via the Search button "from anywhere" in the OS--having to launch an app and then push the button is still more task-intensive than we'd like. But until Microsoft blesses us with this function, Speak To Play is the next best thing.