More from You Tube and this time its more exciting because their new feature makes viewing you tube videos more fun and exciting. And the reason for this is that from the start when You Tube gave 2.5D videos it now gives 3D videos. By tagging a video with "yt3d:enable=true", you can simply enable options which allows you to view a video in 3D modes.
The 3D feature is only available in a beta version, although it can be controlled and activated using special tags.

yt3d:enable=true Enables the view mode. (obviously you've already discovered this)
yt3d:aspect=3:4 Sets the aspect of the encoded video.
yt3d:swap=true Swaps the left and right sources. You may need to add this to videos when the player with fixed anaglyph modes ships. Apologies for the inconvineince.
yt3d:left=0_0.1_0.5_0.9 and yt3d:right=0.5_0.1_1_0.9 These tags are very provisional and most useful for fixing up old videos. They set the source area for each eye as pairs of coordinates x1_y1_x2_y2.

The scale of these coordinates is 0,0 for the the top left down to 1,1 for the bottom right.

I haven’t yet tried or tested this, so will update once I have or if you guys have let me know!