That's really impressive, I didn't realize WP was taking off at that rate (or maybe there is some other reason your app is doing well). This motivates me to finish my guitar tuner app.
Ian, what is your feeling on free/trial/paid apps? Have you considered it with your app, or do you think in-app advertising is the way to go?
For my tuner app, I plan to have a simple tuner, plus an advanced intonation wizard that can help you to intonate your guitar using one of a few available techniques. I'm thinking I can have the free version have just the basic tuner, then upgrade to the paid version to get the intonation wizard (and maybe some other goodies too like a simple frequency counter etc).