Since you don't plan to offer an app for Windows Phone, it would be great if you had a mobile optimized website that Windows Phone users could access. It would also allow app users like myself a way to access if my phone was unavailable, etc.
The current web site is not really usable on a small device.
Cross platform mobile sites are pretty much the norm and I assume way easier to build than a windows phone app. Your cloud site as it is, is not usable from even the largest phones available to day. Please give the people who love your product and use osx and windows an option. Although if you bite the bullet and develop a todo app properly for windows 8 porting it to windows phone is not difficult.