In an effort to move away from Adobe flash video, Youtube is now switching to WebM format.
WebM is an open source format, utilizing the VP8 video codec, Vorbis audio and a file system based on the Matroska container structure. VP8 is owned by Google, the parent company for Youtube.
Unfortunately for Google and Youtube, the Mpeg-LA consortium is examining VP8 very closely, looking for coding that infringes their member's patents. VP8 is similar (though slightly inferior) to Mpeg4 AVC H264, so I suspect that the consortium will find enough evidence to at least start lawsuits and demand royalty payments for this WebM format.
Youtube is still making videos available in H264, as well as their continuing project developing a HTML5 video player, so they aren't putting all the eggs in one (very shaky) basket just yet, but this move to WebM may be a bit premature.