A problem with using the original domain as a subdomain might be that for people not familiar with your service, these URLs look more like a phishing scam than

A problem with using the original domain as a subdomain might be that for people not familiar with your service, these URLs look more like a phishing scam than a shortened URL, quite the opposite of the intended effect ("transparency"): Would you like to go to "http://paypal.twi.bz/c http://paypal.twi.bz/c"?