Guys, it is my understanding that OnApplicationPause() gets called on going background, and again on going back to foreground. To know which case it is, you simply analize the argument passed to the function:

function OnApplicationPause (pause : boolean)
{
   if(pause)
   {
       // we are in background
   }
   else
   {
       // we are in foreground again.
   }
}