Twitter's t.co link shortening service
Twitter has rolled out their new t.co URL shortener.
"t.co currently handles redirections by context and known user agents. We've taken care to preserve original referrers in all contexts where they are reliably provided."
Turns out Twitter is using different types of redirects based on the referrer and the type of browser. Googlebot for example will receive a normal 301 redirect. A human visitor using Firefox or internet Explorer will get the new t.co referrer-changing-redirect.