According to Apple’s press release, YouTube will be available on iPhone on 29th June. YouTube will not be fully supported on iPhone, but will only play H.264 encoded movie clips (not Flash obviously) .

To achieve higher video quality and longer battery life on mobile devices, YouTube has begun encoding their videos in the advanced H.264 format, and iPhone will be the first mobile device to use the H.264-encoded videos. Over 10,000 videos will be available on June 29, and YouTube will be adding more each week until their full catalog of videos is available in the H.264 format this fall.