Any Plugin tutorial ?

I didn't find any tutorials. I used a combination of the Unity reference page, http://unity3d.com/support/documentation/Manual/Plugins.html and the new OpenFeint plugin for Android. The OF download had a sample project that was already set up for JNI and such.

I created a new project in Eclipse and started out small, just a class with log statements so I could see if things were working. Basically you export a jar file from your code and add to the Android plugin folder. From there take a look at the OF cs files to get your bearings.

Trying to build one from scratch was pretty hard, it's much easier just to see what somebody else has done and follow the example.

Good luck