SimpleXML doesn't support CSS selectors, but I know that some PHP and Javascript librairies can convert a CSS selector to an XPath query. IIRC, there's a PHP library that mimicks jQuery's selectors but I can't remember its name. Hopefully it will jumpstart someone else's memory.
Turns out the library I was thinking about doesn't support CSS selectors, but the good news is the Zend Framework has a CSS-to-XPath translator:
include 'Zend/Dom/Query/Css2Xpath.php';
$xpath = Zend_Dom_Query_Css2Xpath::transform('div.class span');
$mySimpleXMLElement->xpath($xpath);