Clipstart can play and upload HD movies. If the uploaded video doesn't need to be modified, Clipstart will upload the original file directly without conversion. If trimming a selection to upload (either to fit in Flickr's 90 second limit or from Clipstart's timeline view), Clipstart will export a copy of the movie to H.264 format using the same resolution, frame rate, and bit rate as the original file.
Note that Flickr Pro or Vimeo Plus are required to view HD movies on the web.