YouTube announced at the VidCon 2010 conference that they are now supporting 4K (4096 x 2304 pixels) video. Here is a short playlist they've put together. Out of curiosity, I tried playing one on my version 1 MBP. It was very jittery even after waiting for the transfer to complete. This MBP can play the 1080p just fine. Of course choosing the 4K option is ridiculous for a laptop and most users.