YouTube added support for playing videos that are bigger than 1080p. "Today at the VidCon 2010 conference, we announced support for videos shot in 4K (a reference resolution of 4096 x 3072), meaning that now we support original video resolution from 360p all the way up to 4096p. To give some perspective on the size of 4K, the ideal screen size for a 4K video is 25 feet; IMAX movies are projected through two 2k resolution projectors," explains YouTube.
There aren't many 4K videos uploaded to YouTube, but you can find some examples in this playlist. To switch to 4K, select "original" from the list of versions that are available. YouTube says that you need an "ultra-fast high-speed broadband connection" and it's probably a good idea to play the videos in fullscreen on a big-screen TV.
For now, the support for 4K videos doesn't mean too much, but it's nice to see YouTube pushing technological boundaries.