Charles Petzold apparently thinks this should change, so he's written an article for MSDN Magazine on how to write a theremin app for WP7.
I know many of you will immediately think "but it's not a theremin!" when you read this, but I encourage you to consider a few things. First, apps like this expose a whole lot of new people to the theremin. Second, after getting a tiny taste for the sound, they're more likely to do want to try the real thing. Finally, phones and mobile devices are quickly gaining ground in the music production space, and it's likely a reality that this type of 'medium' is here to stay for a long time, so why not embrace and extend them as musical instruments?
Let's hope some tech-saavy readers will take Mr. Petzold's cue and build some great theremin-inspired apps for Windows Phone!