YT3D option on YouTube

Uploading 3D videos on YouTube is easier than before with this YT3D option that appeared in July 2009.

To encode videos in 3D for YouTube, prepare your videos in side-by-side format, with both images squeezed horizontally by 50%. Details on the YouTube web site here.

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.
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. Note: Ideally you shouldn't use left and right. For the best quality make the sources as large as possible within the frame and set yt3d:aspect correctly.

The YouTube technical forum about 3D is here.

Notice the "mode drop down listbox" under the video and check the appropriate mode for your 3D screen.