I was recently contacted by 4K Fest to see about screening Sintel in 4K at the event. We're working with them and Ton at the Blender Foundation to make this happen.
Semi-related to this are my efforts toward capturing 4K time-lapse clips. So far I've just been grabbing scenes. The plan is to eventually edit together a narrative of some sort. For now the videos that I've created and tagged as 4K are on YouTube at 4K. Sadly the option to playback at 4K seems to have disappeared, so the best you get is HD.