Since a few weeks ago, it looks like YouTube has disabled RSS feeds for playlists. Is there another way to get an RSS feed for a YouTube playlist?
If you do this often, this is a bookmarklet for extracting the RSS feed:
To use the bookmarklet, create a bookmark and post the above script where you'd normally have your url. Then navigate to a youtube playlist page and click the bookmark. It will extract the playlist id and automatically redirect you to the rss feed.