Should Safari Do Single Sign On

Hi, My clients are bound to active directory and open directory using golden triangle. When a user logs on and opens up Safari it asks them for their username and password. This is a bit inconvenient as I work in a college and this simply confuses both staff and students (they are a little dim!). I've been to other colleges and once you logon Safari opens up and gets online without needing to reauthenticate. Is there something I'm doing wrong? Thanks