Ok, first of all, was ANY of your code or concepts used in the new application? Was your app compiled? Was there copyright notifications within the code you posted, in your space, under your account, on their server?

If yes, yes, or yes, you have the beginnings of a very good case to pursue said person for copyright infringment and theft of intellectual property. Understand, that as soon as a piece of new code hits a machine or piece of paper, it is YOURs by law. The registering of software/code with the copyright office is merely a formallity, and is useful should a problem such as this arise in the future.