Thanks for the suggestion. Unfortunately implementing CSS3 selectors is outside the scope of PIE; it has always been focused on rendering only and will almost certainly remain that way. The main reason is performance -- implementing selectors requires parsing and rewriting of stylesheets, which can be a very time-consuming process.
That said, there are some other great projects out there which implement CSS3 selectors to some degree. Check out the product comparison page at http://css3pie.com/documentation/product-comparison/
-- in particular, IE9.js, eCSStender, or ie-css3.js might be what you're looking for. If you get any of these working successfully in conjunction with PIE for the rendering, I'd be interested to hear if you run into any problems. They should theoretically work together but I haven't tried it myself.