Geckoboard Pricing: Let me know
blog.geckoboard.com
blog.geckoboard.com
One other thing is that if I was using for something serious, I may be worried by geckoboard having access to some of the more sensitive data. I think this could be a big deterent for some.
Overall, you have an absolutely kick-ass product, I suspect that if you dropped the prices a fair bit then even the most casual users would sign up, which should result in greater revenue for you.
Per-device pricing means your users need to make a pricing decision every time they add another device, which they may hate and try to avoid.