gistfile1.js

this.get_start_offset = function () {
   // Get the number of characters before the start of the selection.
   // Thanks, TinyMCE
   var range = this.get_range();
   if ($.browser.msie) {
       // IE support
       // Text selection
       c = -0xFFFFFF; // ooooooooooh, magic
       tr = document.body.createTextRange(); // Create a new blank range
       begin = range.duplicate(); // Create a copy of the range we've selected
       begin.collapse(1); // Collapse it to the left- just a cursor

       tr.moveToElementText(begin.parentElement()); // Encompass the entire chunk the cursor is in
       tr.collapse(); // Collapse it to just a cursor at the beginning
       bp = Math.abs(tr.move('character', c)); // Move to the beginning

       tr = range.duplicate(); // Copy our selection
       tr.collapse(); // Make sure it is indeed a cursor
       sp = Math.abs(tr.move('character', c)); // Move it to the beginning
       return sp - bp; // And there you go.
   } else {

       // blindingly simple ;)
       return range.startOffset;
   }
}