I have posted a demo video clip of Youtube’s new 60 fps support.
To play at 60 fps, you must select 720p or 1080p viewing modes and use the Chrome browser and the HTML5 video player. The Flash player (used in many other browsers, by default) does not appear to support 60 fps and just stops working.

However, if you wish to watch 3D clips on Youtube, then you probably still want to use the Flash-based video player – but it appears we probably will not be watching 60 fps in 3D. It’s possible as we can always upload side-by-side 3D videos in 60 fps mode, but the  HTML5 player will display them in their original side-by-side format – you may have to manually switch your TV or monitor to side-by-side 3D mode in that case, in order to watch in 3D.