Here's a bookmarklet that does this. You'll probably have to squish it all together on one line..

javascript:(function(windowOrLayer)
{
  if (!windowOrLayer)
  {
    windowOrLayer=window;
  }
  for (var f=0;f<document.forms.length;f++)
  {
    for(var i=0;i<windowOrLayer.document.forms[f].elements.length;i++)
    {
      if (windowOrLayer.document.forms[f].elements[i].name == "true" 
        && windowOrLayer.document.forms[f].elements[i].type == "text")
      {
        s=""; 
        for(var c=0;
           c<windowOrLayer.document.forms[f].elements[i].value.length;
            c++)
        {
          ch=windowOrLayer.document.forms[f].elements[i].value.charAt(
+c); 
          if (ch >= 'a' && ch <= 'z')
          {
            ch = ch.toUpperCase() 
          }
          else if (ch >= 'A' && ch <= 'Z')
          {
            ch = ch.toLowerCase()
          };
          s = s + ch;
        }
        windowOrLayer.document.forms[f].elements[i].value=s;
      }
    }
  }
})();
[download]

Update: Fixed error in checking form element name.