An interesting and a pretty good read.
This is one of the most awesome pranks that I have ever played on anyone.
I recently changed my Y! id to a new one but still retain the existing one. Now the prank idea came when I was chatting with a friend of mine on Y! and I rememberd a utility which allows me to start two instances of Y! which is normally not possible.
Now I started chatting with her through both the ids pretending in the old Id to be me and denying that it was me who was using that Id. I pretended that it must have been hijacked since I was not using it anymore. What made it even more plausible was that she did not know about the hack which allows multiple instances of Y!. I said check for yourself and see if you can indeed start the multiple instances.