On:

http://framework.zend.com/manual/en/…

The $conditionalStylesheet is referred to as a boolean rather than a string or false.

"The $media value defaults to 'screen', but may be any valid media value. $conditionalStylesheet is a boolean, and will be used at rendering time to determine if special comments should be included to prevent loading of the stylesheet on certain platforms."

Usage example is:

$view->appendStylesheet("styles.css", 'screen', 'lt IE 7');

Which outputs:

Used as a boolean it would simply output:

Should be updated to reflect that the primary use is to pass a string, but you could pass the boolean value of false.