Matt Cutts posted a video that covers URL shorteners. I thought that in the past he said that URL shorteners do not pass PageRank. He now says that "well-behaved" URL shorteners that use a 301 redirect do pass PageRank.
He does clarify that URL shorteners that use a nofollow link do not pass PageRank, so you will not pick up PageRank from most Twitter links.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin