<p>Is there a way to mount a folder on the hard disk as a device in Finder. The intend here is to provide the user with an easy way to get to a folder that my application uses to store data. I don't want my user to go searching for data in Application Data. I would rather allow them to make this data available as a mounted volume or device in Finder. I would also like this volume or device to be read/write, so that if the user makes any changes to the data files, the changes will get reflected in the original folder.</p>

<p>Is there a way to do this in cocoa, carbon or applescript.</p>

Try looking into FUSE.  You can have all sorts of psuedo filesystems with that.
But I'd caution a little against what you are trying to do.  It may make more sense to just have a button that opens the folder in your application, rather than create a new device.  I personally would find it hard to continue to use an application that does such a thing.  It doesn't really fit with the rest of the available applications.
You could also use an alias to point to your Application Data directory.