Twitter RSS gives you access to the RSS feed for any active Twitter account.
All you have to do is enter the name of the user (no @) to whom you are interested in subscribing in the box, and click the “Get RSS” button.
Then, add the link to your favorite feed reader and you are all set!
Twitter RSS is located at http://www.twitter-rss.com.
UPDATE: The link is dead. You can try the following alternative instead: RSS 4 Twitter.