This concept was originally developed on Ajax Cookbook's Disable Text Selection by Bret Taylor, in 2006. I modified it slightly to use as a method for the Prototype library.

Object.extend(Element.Methods, {
	disableSelection: function(el) {
		with(el) {
			onselectstart = function() { return false; };
			unselectable = 'on';
			style.MozUserSelect = 'none';
			style.cursor = 'default';
		}
	}
});
Element.addMethods();

You can easily call it like this:

<div id="test">abc123</div>
<script type="text/javascript">$('test').disableSelection();</script>

Be sure you include the Element.addMethods(); part, or it won't work. Also, if you're not familiar with the with() statement (searching for "javascript with" on Google is pretty much worthless), read up about it over at the Mozilla Developer Center.