For those who are after some good understanding about Security Implications of URL Shortening Services I recommend Alexander Neumann thesis report, a student at the German Aachen University. In his research Alexander analyzes implications on security and privacy when using URL shortening services. He performs several empirical experiments to verify the threat potential for selected risks. In addition, he analyses the use of URL shortening services in e-mail spam collected in a project over the last seven years.
Download Alexander thesis report from here – http://itsec.rwth-aachen.de/theses/2011%20-%20DA%20-%20Alexander%20Neumann%20-%20Analyzing%20Security%20Implications%20of%20URL%20Shortening%20Services.pdf