Great plugin! Might I recommend integrating it more tightly inside NextGen's admin area?
For instance, there's really only one admin page that's worth noting -- the "Specific Options" page. The "Soon..." page really doesn't offer any functionality to the user. So move it to a readme.txt file in the download, or paste that into the WordPress plugin repository tab for your plugin.
There are two other flash-based NextGen plugins which integrate nicely like this:
Those are good examples. But great work on this!