How to make tinymce paste in plain text by default

For the tinyMCE 3X or 4X things have change a little. now you can do this and it works fine.

tinymce.init({
    plugins: "paste",
    paste_as_text: true
});

I have solved this problem with this code

tinyMCE.init({
...
plugins : "paste",
paste_text_sticky : true,
setup : function(ed) {
    ed.onInit.add(function(ed) {
      ed.pasteAsPlainText = true;
    });
  }
....
})

EDIT: this solution is for version 3.x, for 4.x version read the answer from @Paulo Neves

The problem is that Paste plugin automatically resets plain text paste on every paste. So all we need to do - set it back. The following code should help.

tinyMCE.init({
...
oninit : "setPlainText",
plugins : "paste"

....
});

The definition of setPlainText

 function setPlainText() {
        var ed = tinyMCE.get('elm1');

        ed.pasteAsPlainText = true;  

        //adding handlers crossbrowser
        if (tinymce.isOpera || /Firefox\/2/.test(navigator.userAgent)) {
            ed.onKeyDown.add(function (ed, e) {
                if (((tinymce.isMac ? e.metaKey : e.ctrlKey) && e.keyCode == 86) || (e.shiftKey && e.keyCode == 45))
                    ed.pasteAsPlainText = true;
            });
        } else {            
            ed.onPaste.addToTop(function (ed, e) {
                ed.pasteAsPlainText = true;
            });
        }
    }

So now it always will be plain.

Just ran into this one myself and discovered that as of TinyMCE 3.4.2 you can simply:

paste_text_sticky: true,
paste_text_sticky_default: true

...which was nice.

I think the easiest way would be this:

tinymce.init({
   ...
   paste_as_text: true,
   plugins: "paste",
   ...
});