Ricky Mondello, writing on the WebKit weblog:

By default, Safari no longer tells websites that common plug-ins are installed. It does this by not including information about Flash, Java, Silverlight, and QuickTime in navigator.plugins and navigator.mimeTypes. This convinces websites with both plug-in and HTML5-based media implementations to use their HTML5 implementation.

This is a step in the right direction towards a Flash-free web. If you’d like to experience it now, I suggest uninstalling Flash altogether and using John Gruber’s workaround if you come across any stubborn websites.