I'm working on an application that I would like to authenticate to a joomla site's member database.  I'm fairly new to Joomla and am not sure if there's already a call I could make that would return whether or not the supplied username and password are correct or if I'd have to create a plugin for the joomla site that would allow the application to talk to it.
Ideally, the desktop application would not only authenticate to the joomla site, but have the ability to register new users.
I also want to create some new tables in the Joomla DB to store information specific to the desktop application, and perhaps even allow users to log into the joomla site and see that information.
I hope this is enough information to get an idea of what I'm hoping to accomplish.  Does anyone know if anything like this has been done before?  Does anyone have ideas for how to do this?  I might need to contract out for custom work but would like to know the direction I'm headed...

Why don't you just create a C++ app or whatever that displays a webpage, and then make some HTML pages to be displayed there, which include a form to login etc? Wouldn't that be simple but effective?

I think I need something similar to this Drupal extension: http://drupal.org/project/Services
A REST interface of some sort would do the trick.  Has anyone seen something like this for Joomla?