var ta1 = Titanium.UI.createTextArea({
   id:'idOfADOMElement',
   value: 'Here\'s a new #tweet @joeschmoe',
   keyboardType:Titanium.UI.KEYBOARD_ASCII,
   autocorrect:false,
   textAlign:'left',
   height:100,
   width:255
});

//recalculate on every change
ta1.addEventListener("change", function(e) {
  var text = e.value;
  var tweetLimitStatus = 140 - text.length;
  // tweetLimitStatus is now how many characters remaining in a tweet, can be negative
  // if the tweet were too long
});