Updated: June 6th, 2008
Update: The method described here is obsolete. Youtube and Google released a beautiful API located here: http://code.google.com/apis/youtube/developers_guide_protocol.html. You can do everything mentioned in this article and a lot more using it. Here's a sample: http://gdata.youtube.com/feeds/api/videos?alt=rss&racy=include&vq=BBC+Five&start-index=201&max-results=50
For example, if you wanted to create an RSS feed for the tag "monkey," you would enter: http://www.youtube.com/rss/tag/monkey.rss
For example, if you wanted to create an RSS feed for the user "YouTube," you would enter: http://www.youtube.com/rss/user/youtube/videos.rss
The search by tag approach, however, won't necessarily get complete results because it will depend on user tagging the videos. After digging around a bit (really, just trial-and-error method), I found an unpublished (I think) way to create RSS feeds based on actual search results.
Here it is: http://www.youtube.com/rss/search/[SEARCH_TERMS].rss
For example, http://www.youtube.com/rss/search/steve%20irwin.rss.
The number of results returned seems to be capped at 20. If anyone figures out a param to get this increased, please comment.
In the meantime, if you found this article useful, feel free to buy me a cup of coffee below.