You could try to do the request manually (from the server to be in
exact same condititions) with wget or curl and see what is returned:
curl -v http://twitter.com/statuses/user_timeline/$username.xml
Twitter is rate limiting by account (not the case here since the
request is not authenticatd) but also by IP. If you are on a shared
hosted box, and if someone on your IP is abusing twitter; you may be a
collateral damage.
-Laurent