I think you are looking for:
http://twitter.com/statuses/user_timeline/%USERID%.rss
If% USERID% is the identifier of the user in question.
For example, the URL of my own timeline:
http://twitter.com/statuses/user_timeline/16979198.rss
with 16979198 being my user number.
UPDATE
API Twitter, :
http://twitter.com/statuses/user_timeline/%USERNAME%.rss
% USERNAME% - , .
:
http://twitter.com/statuses/user_timeline/jamiedixon.rss
.rss , .json .atom
API Twitter . :
http://apiwiki.twitter.com/Twitter-REST-API-Method%3A-statuses-user_timeline