Retweets, which add "RT @username: ", push some tweets over that limit, and so they are truncated. As a result any links at the end of the tweet won't work.
In the tweet object you should have a retweeted_status key, this contains details on the original tweet including the un-truncated status, which you should use if you want the full tweet. You can always manually append 'RT @username: ' before that because retweeted_status will also contain the original tweeter's username. Hope that helps.
Thanks, that really helps!