The official RSS feed seems to only support 'favorites' which is a specific playlist, unlike 'likes' (http://www.youtube.com/my_liked_videos) which is where a video is collected once you click the 'thumbs up' icon during or after playback.
It was brought up with the Google Data API team over a year ago, but hasn't seen much discussion/action since: http://code.google.com/p/gdata-issues/issues/detail?id=2345