Clipstart will play and upload HD movies if they don't need to be modified. 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 that is currently a maximum of 540p for 30fps movies, or 720p for 24fps and lower. This limitation will be removed in a future version.
Note that Flickr Pro or Vimeo Plus are required to view HD movies on the web.