Why wouldn't this work?

*in page.ss

<div class=" show$Opt(Count) half$Opt(Half) clearfix">

*in page_controller of Page.php

 	public function getOpt($opt = 'Count'){
	switch($opt):
			case('Count'):
				  return foo;
				break;
			case('Half'):
				  return bar;
				break;
			endswitch;
}

Well I think that u should call for getOpt(...) instead of Opt, and the second thing is that u should a pass a string to a function.

The reason I ask is that I've only seen numbers passed as parameters in all the Silverstripe examples. I will take the string approach and see if that makes the fix.