I bet about the same percentage that know about the long-hold of the home button to bring up the task switcher in Android (not many). I was amazed that my wife, who has had an Android phone for a year, didn't know about that one...
It isn't a task switcher though is it? It just lists the apps you used recently, not the ones that are open. AFAIK you need a separate app that is binded to the home button to get different behavior.
