This is working for me in Chrome. Will test in other browsers now:
$(".editable").on("keyup", function(keyCode){
var linesCount = $(".editable div").length + 1;
$("#linesCount").html(linesCount + " lines.");
return linesCount;
});
For every line, the content editable div will create a new div child. If you count these divs, you get the total amount of lines.
http://jsfiddle.net/hescano/PypeD/
Edit: For FF this actually produces <br>
tags. You could count those.