I think you're in the grey zone where IE just hasn't implemented generated text in CSS. There may be help in the demo files here: http://www.xhtmlchef.com/stc2004.asp . Also, since the French and Italian quotes work, have you tried q:lang(ENG)?
By the way, XHTML 2 seems to be dropping the <q> tag and replacing it with <quote>, which explicitly requires the author and not the browser to provide the quotation marks (http://www.w3.org/TR/2004/WD-xhtml2-20040722/mod-text.html#sec_9.8. ). So I don't suppose we'll see support for CSS-specified quotation marks added to browsers that don't already support them.
Peter