YouTube using H264 initially may just have been because they already have their videos H264-encoded, because that's what they use for the high-res content in their Flash player. They may also be selecting it because of Chrome's support, but I think it's too early to assume for sure that they don't have plans to support Ogg at some point.
Supporting H264 <video> was just a matter of writing some front-end player code (which they already had from their HTML5 demo) and feeding it the same video file they gave the Flash player. Supporting Ogg, on the other hand, would require massive transcoding.
IIRC, the iPhone version is H.264, but at a lower bitrate, so they have 3-4 h.264 files per video:
* 720p version for hdtv videos
* 480p version for non hd web viewing
* High bitrate version for iPhone streaming on wifi
* Low bitrate version for iPhone streaming on 3g or edge.
Supporting H264 <video> was just a matter of writing some front-end player code (which they already had from their HTML5 demo) and feeding it the same video file they gave the Flash player. Supporting Ogg, on the other hand, would require massive transcoding.