Visit BrowserSpy and find out for yourself.
BrowserSpy shows you lots of detailed information about you and your browser: things like the version of your browser, what it supports, and what it doesn't support. Furthermore, it can provide you with detailed information about JavaScript, JVM, Java, Plug-ins, components, language, screen, hardware, IP, cookies, Web Server, FTP Password etc....
You name it, and BrowserSpy shows it!
How do they do it? Basically, they use JavaScript, VBScript and Perl to show you the information. Go through all the categories, first with JavaScript enabled, and then with JavaScript disabled. You will see that without scripts, most of the info will not be visible.
Why should I care? Basically because if BrowserSpy can show the info, a remote site with MPack installed is able to gather a whole lot of info about you, your browser, and your setup and deliver the 'right' payload for a specific browser and its plug-ins.
Paul