I wouldn’t expect much ranking improvement just from having your links on Twitter since the nofollow attribute is placed on those links, and Google is aware of Twitter enough to discount the links anyway. There is some evidence that nofollow links from a site like Wikipedia might carry some juice, which would make some sense with the way that Wikipedia is vigilantly moderated.
Another metric; OpenSiteExplorer rates that Twitter page as a 38/100 with only 4 backlinks, so it appears that the PR is mostly being boosted by the fact that it is on the twitter.com domain.