This thread is a huge eye opener for me.
I thought that just by changing my ip and clearing the cookies I would be leaving no footprint but clearly that is not the case.
I researched more on this. For all of you how want more info here is what I found:
-This can't be done on the server's side. So it can't be done with php etc.
-IMO its quite possible that installed google applications: google chrome, google desktop etc can send out your MAC to their servers. I know how paranoid this sounds, but knowing how information hungry they are its quite possible that they are doing this.
-This can be done in javascript. Its quite easy to implement this to work on IE. But from what I read online it won't work with other browsers. This approach uses ActiveX objects and it will work only on IE run from Windows. So if you use IE disable ActiveX.
-This is a huge problem for the people using automation tools. Most automation tools just run on top of IE.