Seems like many have been using the 2X speed option to hack their way to 60 fps within the html5 version of youtubes player. I'm guessing a few engineers at youtube probably looked at that and some of the twitch streams and figured it would be easy enough to implement.
My guess is they'll try and have the 3rd hobbit movie trailer show up on youtube in 48fps 3D (if you have a monitor that can support it). That will showcase it.
Somehow though, I think the average compression rate will be pretty bad - youtube at 1080p looks about on par with HD on netflix (super HD being one step better). So , sure you'll see 60 FPS so games will look more accurate but it'll probably be capped at 720p and each individual frame will have the bit rate of a 480p 30 fps video frame.
I just don't have hopes, I want to be surprised though, it sure would be nice if youtubers like maximillion_dood who also posts here , could post fighting game videos at their actual native framerate. Shitty bit rate or no , they'd look far better at 60 fps then at 30 I think.