Ultimately, our plan for Thinbox is that the number mapping will be connected to more information than just an e-mail address.

A path we're considering that might have some more obvious use cases: a mapping of phone numbers -> mailing addresses. That way in your mobile postcard app, you can choose a friend to mail a card to, even if you don't have their mailing address in your phone. The first-time recipient would automatically get a text message along the lines of: "Steve would like to send you something. What is your mailing address?"