‌ – zero-width non-joiner (see http://htmlhelp.org/reference/html40/entities/special.html)
On the off chance that this already appears in your text, double it up (eg: ‌‌mytext‌‌
Edit in response to comment: works in Firefox 3. Note that you have to search for the Unicode value of the entity.
<html>
<body>
<div id="test">
This is a ‌test
</div>
<script type="application/javascript">
var myDiv = document.getElementById("test");
var content = myDiv.innerHTML;
var pos = content.indexOf("\u200C");
alert(pos);
</script>
</body>
</html>