There is a myth that adding links on Twitter is useless from an SEO standpoint because it adds nofollow on everything.
Last time when I checked, it wasn’t the case. Though it changed as I used Firefox stylesheet that reveals which links are nofollow and @mention of people’s names were no-followed.
Don’t lose hope yet; adding nofollow by Twitter won’t necessarily hamper your traffic. As long as your posts are popular in terms of retweets and shareable on blogs or websites, they can still get SEO credits. Plus, there are still chances other services like Google or Topsy may not add nofollow to your links.
So post things that people would want to share and retweet to build more links. Also, for the posts whose value you want to pass, remember to use the 301 redirection service.