Perhaps:
if (navigator.plugins["QuickTime Plug-in 2.0"]){
//Code for them having the plugin
}else{
window.location = 'warningpage.html'
//Page containing a warning and a link to Apple
}
This might only work in Netscape though, and instead of searching for a specific version number, you could loop through all the plugins and see if any of the contain the word 'QuickTime' C:\DOS:>
C:\DOS:>RUN
RUN DOS RUN!!