countCSSRules.js

(function() {
    var results = '',
        log = '';
        
    function countSheet(sheet) {
        var count = 0;
        if (!sheet || !sheet.cssRules) return;		
    	for (var j = 0; j < sheet.cssRules.length; j++) {
            if (!sheet.cssRules[j] || !sheet.cssRules[j].selectorText) continue;
            count += sheet.cssRules[j].selectorText.split(',').length;
    	}
    	log += '\nFile: ' + (sheet.href || 'inline <style> tag');
    	log += '\nRules: ' + sheet.cssRules.length;
    	log += '\nSelectors: ' + count;
    	log += '\n--------------------------';
    	if (count >= 4096) {
            results += '\n********************************'
                     + '\nWARNING:'
                     + '\n There are ' + count + ' CSS rules in the stylesheet ' 
                     + sheet.href + ' - IE will ignore the last ' + (count - 4096) + ' rules!\n';
    	}        
    }

    if (!document.styleSheets) return;    
    for (var i = 0; i < document.styleSheets.length; i++) {
        countSheet(document.styleSheets[i]);
    }
    if (!console) return;
    if (log && log.length > 0) console.log(log);
    if (results && results.length > 0) console.log(results);
})();

cleaned up the original code, added console check for ie - but you shouldn't be including this in production code

it doesn't count imported style-sheets, this script is only really useful if you concatenate all of your style-sheets together.