Twitter turned off their RSS support last month. This page gives several ways to create new RSS feeds for Twitter accounts.
Update (October 27, 2014): Here is a cost-free and ad-free Android app that is an RSS feed generator for Twitter.
Update (April 25, 2015): Here is a list of RSS feeds for each of my Twitter accounts, hosted by BazQux.
Update (February 6, 2017): Subscribing to Twitter via RSS is a losing battle. I’ve deleted the rest of this post because it doesn’t work anymore.